This smacks of the same exceedingly poor logic (not yours ZH, but the overarching logic you're explaining) that made some monsters arbitrarily immune to crits in 3e. Simply put, if you want game-world balance and believability, crits (and many other things) have to work both ways equally. A minion can be critted against, and there's no good reason it can't give a crit right back...and if a PC gets unlucky and eats a few crits from some minions, tough. It's the price you might pay for wading in to a sea of red shirts.
Lanefan