I think the problem is this pesky little thing called the first amendment.
"Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof; or abridging the freedom of speech, or of the press; or the right of the people peaceably to assemble, and to petition the Government for a redress of grievances."
And Shariah law, is being interpreted as some type of religous excercise. They are afraid of setting a precedent. Ban one religion, whats to say we don't ban another?
BUT! I thought of an out! All they have to say is , "Shariah law is not in keeping with practices of a modern civilization." Seriously! Christians used to burn people at the stake, but you don't see them doing that anymore, do you? That was an 18th century or earlier practice. Islam needs to catch up with modern humanity (by abandoning barbaric practices as christianity has done), or simply not be admitted to it!
How ya like them apples?!