I assume this will get around Valve's rule of not being able to sell games in countries that don't have PSN support, so it could be seen as a monetary decision. Though, I'm not sure how many big markets don't have PSN support, so who knows.
To me this was a surprisingly consumer-friendly move by Sony. I think they went about the PSN login thing completely the wrong way from the beginning. It should have been an (optional from the start) toe-dip for the Playstation ecosystem. Offer Trophy support alongside the Steam achievements. Offer cross-game save compatibility (even if it's only Playstation -> PC to prevent savegame editing). Offer digital discounts on double dipping, both ways, etc. At least on Steam, they create and control their own game keys, so I would assume they could just sell discounted versions of Steam keys on PSN, similar to how devs sell keys from other stores.
I don't personally see the value in Playstation and PS+ or whatever they're calling it anymore, but there was a lot of room for cross promotion and extending the Playstation platform. Instead, they just made it a mandatory login to help them control banning of players and inflate MAU numbers, which they could already do by adding in the Steam numbers at least. Probably EGS too, I'm sure they have a back end tool to track users.
Anyways, credit where it's due, I'm glad they removed the requirement on a game-by-game basis. I don't personally mind logging in to my PSN account on Steam, but it should not be mandatory, especially for single-player games. There also should be some benefit of logging in, to the players at all levels. That's something they still need to work on, IMO.