Modern religious law it still applicable? Leviticus 20 literally is associated with Israelites. Why is there a necessity for association with modern law?
I have to agree with DJClayworth in that I'm not sure which verse(s) you're referring to from Leviticus 20 have been associated with which modern law(s), but I did want to try and help answer the question you have to the best of my knowledge. 

As a quick intro, many people (even the staunchly devout) have misjudged the Old Testament as being just a historical account of how things started and where Christian beliefs originated. But after reading through the Bible a couple of times and praying that God would show me the message He wanted me to glean from the Old Testament, I realized that the Old Testament has a much greater purpose than simply its historical value. 

In Malachi 3:6, the very first line in that verse says "For I am the Lord, I do not change". And it's in that verse, written in the very last book of the Old Testament, that I finally and truly understood the major value that the OT holds. 

The Old Testament refers to God, specifically, far more than the New Testament (which focuses primarily on Jesus Christ and the Holy Spirit). And in all the instances where God takes action or speaks to someone in the Old Testament, we see God's nature. What He says and tells people to do, plus the things that He does all point to who He is, just as our speech and actions tell others who we are and what our nature is. And while we can learn who God was through the Old Testament, Malachi 3:6 says that we are actually learning and seeing who God still IS to this very moment, because He NEVER changes. 

So in Leviticus 20, some of the verses (such as the ones in regard to Molech) do have a specific and direct relation to the Israelites at that particular time, BUT the nature of those verses also reveal the underlying topics that God cared about then and still cares about today. 

When I read Leviticus 20, I see underlying topics such as loyalty to God, the value of all lives, the dangers of communicating with unknown spirits, the importance of respecting our parents, the importance of fidelity and loyalty in marriage, the seriousness of marriage in general, the concern for good family relationships, the concern for our health and the importance of avoiding sexual behaviours that can negatively affect both our physical and emotional health, etc.

Because God NEVER CHANGES, we can know with complete certainty that the main issues addressed in the verses found in Leviticus 20 are still applicable (as far as God is concerned) to us today. 

Most of the underlying topics from Leviticus 20 are still upheld by many people today for obvious reasons. Most of us know we shouldn't throw our babies in the fire or sleep with our brother . . . or our dog. It's not so much the laws of Leviticus or the laws of modern America that stop us from doing those things. Instead, it's our own conscience that usually puts the kibosh on those behaviors. 

As a final note, to see whether a specific law or principle from the Old Testament was intended to carry through to modern times, look to see if it's mentioned in the New Testament as well as the Old Testament.

Hope that helps!