Well, a 20 isn't automatically a crit even for PCs. If the result of 20 + their mods would actually miss, then it only counts like a normal hit. In my mind, saying that minions never do extra damage from a crit is very similar to this.
Also, when running minions, it's fun not to make it obvious to the PCs who the minions are. Yeah, they'll figure it out eventually, but if you don't have them scream out, "Here! Hit me! I'll go down easy!" then the PCs won't be able to target them as easily. It shouldn't always be obvious to the PCs. And it adds a touch of realism. Heck, every once in a while when you're describing the start of combat go into detail about one of the minions like you would a standard or elite monster, maybe gloss over the description of the actual elite. Then watch the look on their faces when he goes down after one hit! Gotta keep those players on their toes!!