Why does it matter if it's FA affiliated or not?
Firstly, absolutely nothing to do with money. I certainly don't referee for money, I hope the main motivation for anybody is not money, and from how I read the original post the op wants more experience, not money.
The reason I say do FA affiliated is purely the fact that there is support for you, proper sanctions and bans administrated by the FA and some minimum standards enforced by the FA.
Powerleague can do as they want.
Also, while Powerleague will tell you you're ok to show cards as you see fit they mean nothing. You can send a player off in their first game and they're straight back on for the second.
Players also don't have to got through the rotting up and ban procedure they would for FA games.
Imagine a league where the players were poorly behaved to start with, then no ref ever submitted cards, and this is the end result.
Throw in the fact they're literally caged in and there is no support or interest from the management for the referees and that is a toxic mix.
As
@Padfoot correctly said their business model is all about participation and participants paying to rent pitches, plus spending money in the bar afterwards.
That's their focus and it shows.
FA want to get games played properly, end of. And that's their sole focus.