Super Bowl halftime performer detained after unfurling Sudanese-Palestinian flag

This is America.

There's no reason to arrest him, he didn't break any laws.

However, the NFL isn't the government, they're a private organization, they can tell someone that they're banned from their properties and events just about as easily as you can tell someone that they're not welcome at your back yard BBQ (as long as they're not banning them because they belong to a protected class)

They're probably well within their rights here to ban them just because they don't like what he did.

However, I can almost guarantee you that with an organization as lawyered-up as the NFL he signed some sort of contact to be a part of the performance where he agreed to some policy or code of conduct or something that says in some way that performers aren't permitted to go off-script like that, and not only can they decide to ban him for that but they may be able to sue him for breach of contract or something.