I see it a few times a summer. Unless they are acting in a truly dangerous fashion, my thoughts are pretty much along the lines of "oh well", the same as when I see any number of other law violations on a daily basis. Maybe a cop is sitting at the end of the trail and will nab them.

Just be glad we don't have these things legally running around on the bike paths here in America, as the supposedly perfect Dutch bike lane system allows along with mopeds