The international law thing is a tricky subject that I'm not qualified to weigh in on. I haven't heard an educated argument either way. I can see a case for citing decisions from other countries with similar legal systems, but my gut instinct is that it's a bad idea.
In the case of bannng all international law, yeah, there is no reason we should start allowing other countries laws in our courts. We have our own law as a country, we don't need any backseat direction from another country.
The issue is not "backseat direction". The issue is the ability of Oklahoma courts to determine if a judgment against an Oklahoma entity is valid and should be enforced pursuant to 12 O.S. § 718.1 - 12 O.S. § 729.16 and pursuant to Restatement (Second) of Conflict of Laws.
By not allowing Oklahoma courts the ability to reference international law where appropriate, Oklahoma becomes a very unattractive place to do business internationally.
Another issue is cultural defenses other than Sharia Law. Those are more commonly used, and are somewhat successful. But because they do not ring the xenophobic bell that "Sharia" does, they go largely unnoticed.