"Pro" is for riders on a UCI Continental, UCI Professional Continental, or UCI Pro Tour team. Those riders are the only riders considered true "Pro"s
However there are many Elite Cat 1 teams that may use the designation Pro and may in fact even pay their riders. Getting a UCI team license is big bucks and for smaller budget teams, its more worth while to spend that license fee on paying for races or riders. You only really miss out on a few Pro only races.