As far as I know, it looks like a SE requirement, not Sony. You just need the PSN OnLine when you first log in the game. Even if you disconnect from PSN then, you keep online in FF.

Based on this, looks like the only need for this is to confirm if the SE and the PSN acc are the same as linked on MogStation, on the proper system, etc.

So, if we play on SE servers and not PSN, and the Sony ones are used just to confirm your PSN acc, I think SE could stop beeing so greedy and relax this requirement for Ps3/4 players. Both PC and console players pays the same subscrible, but one part need to cross one more gate to get into the game.