As any politician would feed to the news. those Free Speech Zones are for allowing those people freedom of speech without hindrance. It allows for peaceful protest. As, generally, these groups are a minority in a large group of supporters, it would stop supporters from harming protesters and taking away their freedom to peacefully protest.

Now the real reason? So the print and video shots will only show raucous support for the furor. You don't get someone holding up a dead, oil covered duck while crying KILLERS, which would rain on the parade. That plus its easier to avoid getting shot when they are all in once place.