Yeah, the mistake was United's. But the passenger also acted unreasonably. If you act like a two-year-old - even in a situation that, initially, totally isn't your fault - then you may get treated like a two-year-old. And if you act like a two-year-old after the cops start telling you to leave, they're typically going to make you leave - using whatever level of force is required to do so.
So, yes, the initial fault is United's. But there's blame enough to go around.
So, yes, the initial fault is United's. But there's blame enough to go around.