I must admit I don't know the specific details over this controversy. If Sony doesn't want to allow cross-play, then that is okay with me, and I am also under the belief that it's their right to do so.
However, not being able to use your account on other platforms after playing on a PS4 does not settle well with me. Without knowing the details, that sounds both illegal and not possible. I'm wondering: If you played Fortnite on PC initially, then bought stuff on your account, then played on PS4 with that same account, then tried logging in on Switch, would you be blocked? The accounts themselves belong to EG, which makes me wonder how Sony is able to prevent this in the first place? Did Sony and EG strike a deal together where the only way Sony was allowing Fortnite to be playable on PS4 was if they gained the right to prevent other platforms from using the account? If that's the case, then I would only hope that it's made very clear to the players who spent money on the game. I would also hope that there would be a support number you can call to get things sorted out manually, as there are bound to be some people who got screwed over (most likely lots of people).
In summary, it's hard to know exactly how much at fault Sony is in this situation. The fact is, there are many, many people who got screwed over this, and that's the shitty part.