It's one thing to steal or destroy a sign just for fun. I'm not sure what MAKES this fun, but kids (presumably) have been doing things of the sort for all of eternity. However, torching a sign as it hangs attached to a wooden fence is twisted and dangerous. One gust of wind and the fence, and possibly some homes, could have burned to the ground.
If you recognize this firebug as someone you know and love, seek professional help before someone gets hurt.