Unless you are an uncaring psychopath you should pay servers in the US for bad service as well - everyone should be paid for the labor they do. If they aren't doing an adequate job they should be reprimanded or fired. That's not your job though, that's their boss's job. You don't know if they are just having an off day or the problem you had was caused by someone else.
Furthermore, most places pool tips so you're probably docking the pay of a really good server.