Don't see what the big deal. They don't want you playing with the competitors player base. It's a business decision. Just like MS doesn't offer real free to play....business decision.
Do you have a netflix account?
I assume if you do you have it tied to multiple devices, your consoles, your cellphone, your smart tv, etc.
Now imagine you log in to your netflix account in your cellphone but it says "can't access this account because it is restricted to your PS4", oh shit, you forgot you watched some movie on the PS4! now if you want to have netflix on all your other devices you need a second account.
That's exactly what's happening here, Sony is punishing consumers who used their Fortnite accounts on the PS4, fortnite is a 3rd party application that is supposed to have portability among all platforms it is available at, sure sony can block crossplay all they want, but blocking your Fornite account which should be compatible with all platforms is not okay and it's vile, specially if you have invested money on fortnite, now if you want all the shit you have on your PS4, which i repeat, should be tied to your fortnite account and thus all platforms, you will need to pay again.
This is what is being reported, this is why there is outrage.