"Our rules" also include the First Amendment. The only lawful imposition of Sharia law going on in the US is individuals choosing to settle disputes according to its rules--that is, choosing the rules for contract arbitration, whatever the subject of the contract. Would you forbid individuals from contracting as they see fit?
Forcible imposition is, of course, already prosecutable as assault, battery, (attempted) murder, etc. No need to make it double-secret-illegal.
I don't care if Muslims use Islamic Law to settle civil disputes within their communities as arbitration. I have a BIG problem if they attempt to use if for anything covered by a criminal statute. It's an important distinction.