Since you mention myths, I will have to inform you that "Christian law" is precisely one of those myths as well. Christian law and Unicorns both fall into the same category. There is Old Testament Mosaic Law which was specifically ruled not applicable to Christians and isn't even a usable codified legal system. I guess you could say Catholic Canon law was a legal system as it was a codified system; but it wasn't a fully formed legal system for a state. And of course Protestants would have something to say about that being made the law of the land.
On the other hand Sharia is a codified Islamic Law with 4 schools of jurisprudence. Islam the religion and Islam the political and legal system have been totally integrated since its beginnings with Muhammad. Many Muslim majority countries have Constitutions that state that all laws are based on Sharia or can't conflict with Sharia. Depending on which school of jurisprudence, Sharia has a penalty of death for apostasy (leaving Islam), blasphemy and of course the ever famous stoning for adultery. While the percentage of Muslims in the United States that want Sharia to be the law of the land is low, there are many countries where vast majorities of the population want Sharia to be the law of the land.
Here is a Pew research poll about Muslims beliefs about Sharia that got famous rather recently with the whole Bill Maher/Ben Affleck/Sam Harris debate. It shows that, while the percentages of Muslims in some countries that want Sharia to be the law of the land is low, there are many countries with very high percentages that desire this. And in counties that do, there are often high percentages that feel death is the appropriate penalty for apostasy and blasphemy. And this poll didn't even include Saudia Arabia or Iran because you cannot conduct a poll there - places where full Sharia law is the legal system of the country and it would likely mean imprisonment or death for Muslims to disagree.
I have no disagreement with you that in the United States has zero chance of this happening, and I don't know of any Muslim here that would want that (although the percentages locally are not zero - and we get immigrants from places like Somalia who do hold these extreme beliefs). But while these percentages collectively are not a majority of Muslims, they do represent that hundreds of millions of Muslims world-wide do favor an Islamic state and death for insulting the Prophet among other extreme beliefs.
But it gets rather tiresome constantly hearing the "dangers" of folks like Rick Santorum, Mike Huckabee, and Pat Robertson. I don't think Huckabee belongs with the other two as he is generally pretty reasonable. But none of these "dangerous" people are calling for an overthrow of the United States to be replaced by some type of Christian theocracy under the rule of a non-existent Biblical Law.