That seems to be an issue with Fortnite itself. I don't see how Sony could stop anyone from logging on Switch on a game that belongs to a third party. At best it's Nintendo that should be blamed for not letting people on THEIR platform log on a game for whatever reason.

But I don't even think that's the case, it must be a purely Fortnite issue because I assume the account you can't log on is a fortnite account and not a Sony nor Nintendo account.

Here are the messages you get when you try to log into an Xbox or a Switch, with and account that logged into PS4.

"This Fornite account is associated with a platform which does not allow it to operate on Xbox. Neither the Fortnite website or Epic Customer Service are able to change this. To Play Fortnite on Xbox, please create a new account."

or

"This Fornite account is associated with a platform which does not allow it to operate on Switch. Neither the Fortnite website or Epic Customer Service are able to change this. To Play Fortnite on Switch, please create a new account."

The general consensus across the entire internet is that it is Sony's fault. The only time the issue ever comes up is if someone tries using an account that has logged into a PS4 at some point. Never when dealing with any other system, PC, phone, ect.

PS4 is able to cross play with Mobile and with PC.  It is not able to cross play with Xbox or Switch (specifically speaking about Fortnite here)

So Fortnite is basically this:

PS4+PC+Moble

Xbox+PC+Moble

Switch+PC+Mobile

And it might even be: Xbox+Switch+PC+Mobile

Its a bit silly.  Sadly, I started Fortnite on PS4 a few months back, so I am stuck on my island or with multiple accounts.

Sony seems to be returning to its arrogant, anti consumer ways. I much prefer when they strive to try and get people on there consoles.

Even if it's Sony's fault, Sony has NO POWER WHATSOEVER on what happens on a Switch or an Xbox so at best Sony asked Nintendo and Microsoft to block Fortnite accounts created on PS4 and both Nintendo and Microsoft are then accomplices to this. So worst case scenario all 3 brands are just as guilty, though Sony could be the one asking for this cause we all know Sony does not want to go for cross-platform gaming.

Saying that Sony blocks an account on Switch for Fortnite sounds as ludicrous as saying that Sony blocks Forza on Xbox should Forza not work on Xbox for some reason. I could conceive this if Fortnite was a Sony IP, but it's not so again only way to get this blocking to work is with the complicity of both Microsoft and Nintendo... And also the owners of the Fortnite IP must be accomplices.

The other way around would make more sense: Someone creating an account on Switch for fortnite would see it not work on Sony cause Sony can block anything it wants on Sony hardware, obviously but Whatever machine something is blocked on is THAT machine's fault primarily.

Congrats, you are the only person in the world that thinks Sony is not the guilty party in all of this.

I suppose you also believe Sony when they say they don't like Crossplatform, because they need to protect the kids from the evil people on other systems.

Here are the facts:

Fortnite is crossplay between PS4, PC and Mobilie.

Fortnite is Crossplay between Xbox, PC, Switch and Mobile.

You can log into your epic account on the Xbox, PC, Switch, Mobile and get all your items, progress, ect with no problem. If you have ever logged into your Epic account on a PS$, you can no longer ever use that account on a Xbox or Switch.

Xbox, Switch and PC can all crosssplay with Minecraft and Rocket League. PS4 is PC only.

If you somehow can read all of that and believe that somehow it's Nintendo or Xbox's fault, then...

That's not how it works, I created my account on PC.
Sony just said no no

Yeah, for that reason it won't last once their total share of the market decreases further, but for now it's actually precisely that reasoning that has lead them to refuse cross platform play. When you have such an enormous share of the market it just makes sense to let everyone know that if they want to play with their friends (which will more often than not be playing on the PS4), you have to get a PS4 and not one of their competitors. 

Like I said, when you're on top you can use slimy tactics like that to try to remain on top, and lord knows Microsoft and Nintendo would probably do similarly in the same position. Once their share of the pie shrinks further, though, they'll abandon that approach.

Wait what? They block your account on other consoles?

Always interesting (and good for a laugh) to see the mental gymnastics people come up with to try and blame Nintendo for someone else's policies.