Why assume the anonymous diner was a Christian? He could have been a Muslim.
In theory, yes, but 1. This is Kansas and it is not like NYC or Boston or Chicago or LA or even a typical large Texas city where Muslims are in particularly high concentrations at all 2. If he was Muslim, he would not only have refused to tip but also spend the whole meal shouting obscenities, throwing stuff at the waiter and maybe even trying to assault him, creating a whole lot of further publicity that would be very hard to ignore.
No, the alleged note blamed God. Muslims don’t believe in God.
exactly