What is the basis that some Protestants teach the non-necessity of baptism for salvation? I have heard many Protestants say that "baptism is not necessary for salvation". They follow Paul I guess, who says all works including Baptism or circumcision are optional. But having read the following I am not sure how Protestants justify not doing baptism.


  Jesus answered, "I tell you the truth, no one can enter the kingdom of
  God unless he is born of water and the Spirit.
  John 3:5
  
  16 And now what are you waiting for? Get up, be baptized and wash your
  sins away, calling on his name.'
  Acts 22:16
  
  he saved us, not because of righteous things we had done, but because of his mercy. He saved us through the washing of rebirth and
  renewal by the Holy Spirit,
  Titus 3:5


Great commission was given to baptise disciples in all the nations: 


  Therefore go and make disciples of all nations, baptizing them in the
  name of the Father and of the Son and of the Holy Spirit,
  Matthew 28:19


Many Church Fathers affirmed the necessity of baptism for salvation:

Origen:


  "It is not possible to receive forgiveness of sins without baptism"
  (Exhortation to the Martyrs 30 [A.D. 235]).


Hermas:


  "‘I have heard, sir,’ said I [to the Shepherd], ‘from some teacher,
  that there is no other repentance except that which took place when we
  went down into the water and obtained the remission of our former
  sins.’ He said to me, ‘You have heard rightly, for so it is’" (The
  Shepherd 4:3:1–2 [A.D. 80]).


Tertullian:


  "Without baptism, salvation is attainable by none" (Baptism 1 [A.D. 203])


Hippolytus:


  "Perhaps someone will ask, ‘What does it conduce unto piety to be
  baptized?’ In the first place, that you may do what has seemed good to
  God; in the next place, being born again by water unto God so that you
  change your first birth, which was from concupiscence, and are able to
  attain salvation, which would otherwise be impossible. For thus the
  [prophet] has sworn to us: ‘Amen, I say to you, unless you are born
  again with living water, into the name of the Father, Son, and Holy
  Spirit, you shall not enter into the kingdom of heaven.’ Therefore,
  fly to the water, for this alone can extinguish the fire. He who will
  not come to the water still carries around with him the spirit of
  insanity for the sake of which he will not come to the living water
  for his own salvation" (Homilies11:26 [A.D. 217]).


Even Martin Luther affirmed the necessity of Baptism


  "Baptism is no human plaything but is instituted by God himself.
  Moreover, it is solemnly and strictly
  commanded that we must be baptized or we shall not be saved. We are
  not to regard it as an indifferent matter, then, like putting on a new
  red coat. It is of the greatest importance that we regard baptism as
  excellent, glorious, and exalted."
  Large Catechism 4:6


So it baffles me how many Protestants teach that it is not necessary for salvation. How would they address the above? What is their biblical basis for this belief?

It seems to me that many people accept the free gift of Jesus on the cross because they do not want to go to hell. Even though they may have some doubts, they choose to believe and become Christians because the stakes are so high. Protestants exhort them not to trust in universalism but in fact repent of their sins and ask for forgiveness in order to get saved. Similarly, then, wouldn't the same arguments apply to baptism? Where is the difference? The same Church fathers and universal traditions in the first few centuries they use to establish the trustworthiness of the books in their canon also affirms the necessity of baptism.  And yet they confidently tell people it's totally optional?
Why do Protestants teach the non-necessity of baptism for salvation?

Those who see baptism as something additional to salvation might use the following verses;


  Luke 23:43  And Jesus said unto him, Verily I say unto thee, To day
  shalt thou be with me in paradise.


The thief on the cross was not baptized.


  1 Corinthians 1:17  For Christ sent me not to baptize, but to preach
  the gospel: not with wisdom of words, lest the cross of Christ should
  be made of none effect.


Paul did baptize a few but was not sent to baptize which would indicate that the act was peripheral to salvation.


  Ephesians 2:8-9  For by grace are ye saved through faith; and that not
  of yourselves: it is the gift of God:  Not of works, lest any man
  should boast.


Baptism can be considered a "work" (something a person does) and therefore inconsistent with salvation by faith.


  Acts 16:30-31  And brought them out, and said, Sirs, what must I do to
  be saved?   And they said, Believe on the Lord Jesus Christ, and thou
  shalt be saved, and thy house.


Although baptism can follow quickly, the actual process of salvation is never cited as requiring baptism.

The often cited verses used to favor baptism can also be seen in a different light.


  John 3:5  Jesus answered, Verily, verily, I say unto thee, Except a
  man be born of water and of the Spirit, he cannot enter into the
  kingdom of God.


When Jesus talks to Nicodemus, the context is physical human birth and spiritual (born again) birth, the "water" and the spirit.


  Acts 22:16  And now why tarriest thou? arise, and be baptized, and
  wash away thy sins, calling on the name of the Lord.


Ananias came and restored to Paul his sight. He then told him the purpose Jesus had for him. He then summarized for Paul that he should be baptized, wash away his sins, and call on the name of the Lord. This is not necessarily a description of a salvation process.


  Titus 3:5  Not by works of righteousness which we have done, but
  according to his mercy he saved us, by the washing of regeneration,
  and renewing of the Holy Ghost;


The "washing" here is the baptism (immersion) in the Holy Spirit that happens when a person is saved.


  Matthew 28:19  Go ye therefore, and teach all nations, baptizing them
  in the name of the Father, and of the Son, and of the Holy Ghost:


This "commission" was given to the disciples in anticipation of the Kingdom activities expected of the nation of Israel. If it were a universal activity, Paul would have been instructed to include it in his apostleship to the gentiles. In any event, it is still not described as an integral part of the salvation process.

Many find the views of church "fathers" to be compelling, however as far as the Bible goes, it is difficult to find any specific instruction of baptism as essential for salvation.