1. They made them up ahead of time in order to target those who show up at the rally with the racist signs/shirts/etc that you so vehemently decry, while operating under the flawed assumption that people with such signs are democratic "shills".
2. That they actually planted this guy themselves.
I make no assumptions about either possibility, though.
You, however, are stating that it must be choice two. Unfortunately for you, your argument defeats itself because you are saying the racist signs are at every rally in abundance on one hand, but that these people couldn't have had the foresight to create signs for the express purposes of trying and counter the racist-types of signs before they went to the rally on the other.
Thus, your argument relies entirely on fallacy, and is therefore, totally illogical.