Why undermine the offender registry by placing protesters on it?

The World Naked Bike Ride is today for the northern hemisphere. It is a protest against overuse of co2 emitting vehicles. Here' my beef. I read some article that stated Boulder, CO police would arrest participants, and they would have to register as offenders if convicted. Here's my beef. Even if you disagree with protests, and you think the law should penalize them, why not have the penalty be a fine? Doesn't this sort of thing undermine the integrity of the offender registry? People will be able to say "sure, that guy's a offender, but maybe he really didn't do anything wrong." I'd like to be able to know that the registry is reserved for REAL perverted criminals. BTW, I personally think the bike ride should be protected speech.