Isn’t this the deal we’ve made? You can, legally, deny someone access to private property for any non-protected reason.
On the other hand, if the private property was constructed with public monies, which MSG probably was, that’s an interesting debate. Should the involvement of public money confer first amendment protections of some sort? I think it should.
Edit: You can’t use “my” money to build something and then ban me from it because I said something (non-protected) about the CEO of the company that owns it.