> As a foreigner in the US, I've made the faux-pas of assuming waiters and waitresses were being paid a full salary by their employer.

This depends on the state. For example, in California, servers get state minimum wage, at least ($16.90/hour). On the other hand, most "red" states use the federal minimums: $2.13/hour for servers (!) and $7.25/hour for non-tipped employees.