If you go to a music concert or sporting event to heckle the performers or players, they can kick you out. You also have to have a ticket to get in. Attendees to events you noted have tickets to get in, and in the case of CPAC paid money and filed paperwork to attend. Anytime someone rents a hall for any event, they can determine the criteria for attendence. It's illegal to crash a wedding or prom too, by the way. There is a difference between private and public events.