They are not being registered to vote. Most of these currently undoccumented workers will never become citizens. Many dont want to be. What the legislation does is give them the right to hold a green card, work in the United States and travel back and forth between the US and Mexico.
It eliminates the fear that someone will turn them in, forcing them to leave the country and give up all their possessions and possibly be separated from their kids who are US citizens by birth.
I just came from a town meeting in AMherst Mass. I wish I knew of the other towns that Ihave heard of, but in AMherst Mass, illegal aliens are allowed to vote in all elections.