In the past, I have supported the right to open carry while questioning the wisdom of doing so in urban areas for a variety of reasons. Now, I'm beginning to re-think my position on open carry and support this type of demonstration (and I do believe it was a quiet demonstration). The reason the police questioned these people is because they were exercising a right, a right the good people of Wisconsin are apparently on the brink of losing through failure to exercise it.
The police action is equivalent to what is known as a "heckler's veto" where a heckler tries to veto a person's right to free speech by causing such a ruckus that the police stop or arrest the speaker. Wikipedia has some general information about this --
http://en.wikipedia.org/wiki/Heckler%27s_veto.
Here, there does not seem to be any immediate danger or threat to the public safety caused by the "heckler" so that the charges by the police are patently unwarranted.