Why is a distressing spirit said to be from God? 1 Samuel 16 reads:


  14 But the Spirit of the Lord departed from Saul, and a distressing
  spirit from the Lord troubled him. 15 And Saul’s servants said to him,
  “Surely, a distressing spirit from God is troubling you. 16 Let our
  master now command your servants, who are before you, to seek out a
  man who is a skillful player on the harp. And it shall be that he will
  play it with his hand when the distressing spirit from God is upon
  you, and you shall be well.”


and: 


  23 And so it was, whenever the spirit from God was upon Saul, that
  David would take a harp and play it with his hand. Then Saul would
  become refreshed and well, and the distressing spirit would depart
  from him.


Why is the distressing spirit said to be from God, as God is good?

EDIT: This C.SE question addresses the question "How can a Wholly Good God Deliberately Create Evil?" In this question I am looking for the reason why the distressing spirit is addressed to God, and not, for example, to satan.
God is Completely Sovereign

All events can be credited to him in the sense that nothing happens without him at least allowing it, even if he is not exactly the agent of delivery. This is not necessarily to say that what happens is his highest preference (e.g. he would have preferred that Adam and Eve not sin), but it conforms to his (permissive) will. He is willing to permit us to sin and hurt other people, even though he could prevent it.

Job assigns responsibility to God for all his adversity. When God rebukes Job, he doesn't correct him for such thinking. He only says, "Who are you to question me?" Later, he says that Job has spoken of him correctly (Job 42:7, compared to his three friends, at least). Even though we know that Satan is the malevolent agent who wanted to destroy Job, he was completely bound by what God would permit.

It is as true to assign responsibility to Satan for performing the action as it is to assign culpability to God for allowing it, for God could certainly prevent/stop it if he wanted to. God is in no way powerless to have his way.