I got the sense while watching that his security people and Phoenix PD had requested that he not pardon Arpaio during the rally unless he wanted to light a match to the crowd outside and start WWIII.
I got the impression that Trump's crowd could have made the protestors flee in terror if they but turned in their direction. I think the police were trying to err on the side of caution, but there was no threat of any serious violence to anyone but the obnoxious protestors, and that would have been just fine with me.