What do they even gain from doing this with a single-player game?
It could also be an anti-piracy measure, though I'm dubious about how well it will work. Or it could simply be because the company they've got doing the porting doesn't have access to the code that does the PSN check, so they can't remove it safely.

P.S. I was going to point out that, of those 100+ countries, most probably wouldn't have much use for the game anyway due to language barriers but this game supports a LOT of languages!
 
It could also be an anti-piracy measure, though I'm dubious about how well it will work. Or it could simply be because the company they've got doing the porting doesn't have access to the code that does the PSN check, so they can't remove it safely.
I don't think it's the latter since other games they've already ported would also have used it on PS4. If it's the former, I doubt that would be very effective.