I don't know anything about the law but if someone is using their phone to record someone else without their permission while they heckle that person, it's totally fine with me if the first person gets the phone slapped out of their hands. Have some decency.
I don't know what country you're from, but In the US we have many civil rights and freedoms. One of those freedoms is protected under the first amendment of the US constitution, it's called "freedom of the press" which protects citizens right to film/record in public places. (the arena is private but their rules allow filming). it's not just legally ok to do, its a civil right in the US. All the state's allow filming in public. If you slap someone's phone out of their hands, that generally satisfies the conditions of assault, vandalism of property, etc
In other countries like north Korea, activity like filming in public may not be allowed. But we don't live in a oppressive dictatorship
Photographing of private property that is open to the public (such as a mall, a concert venue, or, indeed, a sporting arena) can be restricted by the owner with or without formally posted notices. That is, if this person is instructed to stop videotaping, he should comply and his First Amendment rights will not be violated. And yes, if he refuses to comply he can be booted and if he refuses to comply again -- be guilty of criminal trespassing.
The rule you're describing applies to public properties and tourist attractions.
2 things to add to your post:
1) You are correct, you have no irrevokable right to freedom of anything on private property, but I seriously doubt he was asked to leave before he was assaulted... and
2) Even if he was asked to leave and didn't comply (which there is no evidence of this at all), Harden still can't hit him.
Any way you slice it, Harden is in the wrong. I know there are a lot of "tough guys" around life who act out violently when someone insults your <insert beloved figure>, but it doesn't provide for good society. Best thing to do is to ignore someone until there is a physical threat. Generally speaking, it takes people to reach full mental maturity (about 25) to accept and understand this and stop getting into fights over words. James Harden is 28. He should know better.
Given the NBA's history with player/fan violence, I should hope that Silver strikes down hard here.