Yeah, it happens occassionally, but not from good waitstaff. It's not normal or appropriate behavior, but it still happens once in a while.
It's not about whether or not they have a "right" to do it, either. People have the "right" to say whatever they dang well please to each other, but that doesn't make it okay, and it doesn't mean they won't face consequences. (customer complaints, firing, overall decline in customer base, etc.)