Then again, you could (and probably do) subscribe to the "free market" theory of PuGing: if you think that's silly, don't join that raid, and eventually people who want those requirements won't have anyone to run with (and if they do have people to run with, then apparently even those who have conquered Naxx 25 still want to go back and run 10, for fun or badges or offspec gear or whatever). But that does leave out people who really do need to (and can) run the instances -- just because you don't have a character full of gear from Naxx 25 doesn't mean you can't perform respectably in Naxx 10.
But people who PuG regularly can be pretty cynical, I'd imagine, and so most of them would rather be safe (and take people who are overgeared) than sorry (and stuck in a bad group with people who don't belong). All the more reason to find a solid guild, or at least make your reputation in a raiding group. It does seem a little out of whack to require higher achievements than the content you're raiding, but as long as people are still joining those raids, PuGers will keep requiring that level of gear.