It may be a running race, and I'm not sure what the French law would be, but by choosing to participate in this event unregistered Hirt was essentially trespassing. Unless the course was open for others to run on during the race, which it would be hard to see as a possibility, there's no way to justify him being out there. Being forcibly removed from the course was the lightest that should have been done - he's lucky he wasn't arrested. It's no different from any other sport's playing field. What happens when somebody runs out on a soccer pitch? How about those idiots who decide they want to run the bases during an MLB game? Those dopes get popped by the grounds crew or stadium security.
After the debacle in the 2004 Olympic marathon that possibly cost de Lima the gold, any time somebody not in the race decides they want a piece of the action, I hope race officials take swift and decisive measures.