The shirts are a sign to drivers regarding your intentions and why you are riding the way that you are. It tells them that there is a set of state laws (in Idaho) that allow for you to yield to a stop sign and stop at a street light showing red and then go. In other states it is a cyclist's wish that the local laws be changed to match Idaho's.
The idea is that you are communicating with the drivers coming up behind you, which is almost always a good thing, and that you are asking them in a friendly way to let you ride following a set of rules that makes more sense for everyone. You'd rather that they not get on your case for appearing to be caviler about the rules of the road. And that you would appreciate their understanding.
The shirts display a political statement and foster, through social means, a political movement. We want the laws changed and this is a measure toward changing the political climate. Many cyclists follow these rules anyway and you are not going to change that behavior, so we might as well take advantage of it.