Using sharia as an example if there is no law that plainly says sharia is illegal in this country its only a matter of time before some liberal says that we should use it.
The 1st amendment of the Bill of Rights already makes all religiously based legislation illegal. That encompasses sharia law as well, making it unable to be passed. It also encompasses laws specifically dealing with Sharia law - because that law itself is a law dealing with religion.
It's like 2 kids in the backseat where the dad tells them both to shutup and be quiet. If kid #1 starts yammering again then kid #2 doesn't get to start yelling at him about how dad told them to be quiet. He told them BOTH to be quiet. That means no talking, even if the talking is telling the other not to talk. Kid #2 needs to sit back and let dad handle it, lest he break the rules as well.
The same applies to the laws against sharia. The same law that makes those laws unconstitutional also renders them completely redundant. Sharia is already illegal.
As to the topic about Drs and guns, I'd say a better solution would be to as several have suggested, vote with your wallet. If they ask a (non-friendly) question about guns, simply go elsewhere and patronize a different doctor.