Can't really tell how thick the ice is. It can't be very thick, as it was in the 50s less than a week ago. Not to mention, it's a river, so there's moving water.
They do it every year, though, and no one has died that I know of. The water is very shallow there, anyway. Still dumb, IMO.
EDIT: I misread your post. Didn't see the law part of your question. I don't think there is a law. If it was on state park land and there was a do not go onto the ice sign, I'm sure that would be different.