I'm not sure what court case and legislation you're referring to, but as a matter of contract law, USians are free to adopt whatever contractual obligations (with certain exceptions) that they want. Because freedom.If this is such a straw man, then please explain why certain persons in tandem with the ACLU felt the need to go to federal court to defend the use of sharia in court, and succeeded in having Oklahoma's state law and also a state constitutional amendment duly ratified by referendum tossed out. I doubt they would go to so much trouble without the intent of making use of this ruling in the future.
If I sign a contract saying that I'll only serve halal/kosher food, that becomes a legally binding obligation. If I don't, then I can be sued. That there are religious texts that make those designations important don't really matter.
I am familiar with a case once involving a Protestant denomination arguing about who had the rights to certain real estate. The operating agreement they signed incorporated a byzantine (pun intended) set of rules on how issues were resolved within a congregation. The courts had to figure out how to interpret the religious document within the context of a legal contractual dispute.
As for legislation - any legislation that favors one religion over another is likely to be unconstitutional.