It has always been player led. If a player is making his sole income from playing the game then he ceases to be an amateur
Off the top of my head I can think of Six APATers who have played our national live events and have since become pros and ruled themselves out of eligibility
Above that, there have been occasional questions about others, and we discuss the situation with the player privately to make sure as far as possible our fields contain amateur/recreational players only
That does not stop an APATer who play festival events or qualifies for EPTs etc playing APAT events, if they have other sources of income. Again, this is the case for quite a few players