Re: Strongly discouraged
They might discourage you from buying a larger one in the hopes that you instead buy a new Surface with a larger disk. Or to make you concerned about getting a third-party one when they eventually become available. They could be hoping that people think "You can't even replace the disk with one of a different size. No way this one from another manufacturer will work." and then you'd buy from them. I don't think it will really help much, but they could be thinking that.
Your PlayStation comparison is possible, but I doubt it. Before I say too much and look stupid, I am not a gamer and know relatively little about the device, but I read this description of the SSD. Based on that, it looks like they've designed a drive for really fast reads so they can load large game data quickly. That's not the kind of drive you find in a tablet-laptop. Among other things, it looks like the fast speeds from Sony's drive come with a higher power requirement, which isn't ideal in a battery-powered device. Even if MS does have unusually fast drives, Windows can run on spinning drives today, so it clearly doesn't need it. They probably would be more successful at selling these replacement drives if they said "Other drives will work, but these ones are really fast so you'll like them better". I don't think that's the case.