Yes, you are correct. This is a concern that is shared between many groups and activists. On one hand, people are willing to support anyone that is trying to address the problem in a civic way. On the other hand, people are concerned about who will show up during Minuteman and something may go wrong. It is a risk as are many things of value in life. What is the old saying? "No guts no glory?"
That being said, I think the best way to deal with the danger is to give people information and let them make up their own minds. There is enough censorship of immigration issues by MSM and people are very alert to the same from discussion forums.