This is fair, but it's an oddly mixed bag on Playstation for legacy reasons.

Free-to-play games can be played online without a subscription, but paid games require a subscription - I imagine as a holdover from pre-Fortnite days and cross-platform play.

I'm a subscriber for the game catalogue and only really play online multiplayers that are F2P anyway, but yeah it doesn't seem at all justifiable. It's a strange decision too - I can't imagine the subset of people who are NOT subscribers for the monthly games and/or catalogue AND are playing games online that are NOT f2p is big enough to be a significant impact to their bottom line. If anything it would be a deterrent from choosing the platform.