they don't "have to". its common courtesy.While yes, theres no explicit law that says restaurants must provide free tap water, most counties point to the International Plumbing Code for buildings and establishments to follow. And guess what the IPC requires? Free access to water.
there is no law in the US saying that restaurants are required to provide water.
https://www.eposnow.com/us/resources/are-restaurants-required-to-provide-water/
this = wrong