So... a player came to my table with an Expert character. I looked it up, nothing in the additional resources except that everything in the Core Rulebook is permitted, with specific exceptions. So with no apparent reason to disallow it, I let him play that character.
Did I miss something that would ban NPC class characters from PFS? At least they are not overpowered, and they are capable enough that if well optimized, they would still certainly wipe the floor with a badly optimized PC class character.
As for the question "why", I can only say: "because we can". (Or can we?)