Most candidates screen the crowds for their much smaller rallies so that you have a very tightly controlled audience. Take Shrillary for instance, I believe her people handpick the people who attend her rallies and actually invite certain people to attend that they know they can count on to deliver an enthusiastic reception. When the campaign comes back to a certain town, the same people are invited again.
I seriously doubt that any Freeper would get even close to a Shrillary Clinton rally.
On the other hand, Trump basically opens the gates and lets anybody with a ticket in. Thus it is very easy for protesters to gain access.
Just food for thought.