Mirror Shield doesn't reflect Trebuchet becaue it doesn't actually target the opponent. Simple as.
It targets the opponent in the same way that UG does.
Basically, I don't like exceptions. As it stands currently, mirror shield blocks all direct damage except Trebuchet (and poison, but that's a different story).
I wouldn't have as much of a problem with it if there was something to indicate that it functioned this way, but I'd rather the game was just consistent.