I think you've confused what this means.
Critical hits don't prevent negative stat changes from happening, all they do is exclude them from the damage calculation. So you will still receive the Special Attack drop from Draco Meteor, but when you use an attacking move and get a critical hit, that Special Attack drop won't reduce the damage dealt.
So the general idea is that they ignore negative stat changes, but they don't completely stop them from happening in the first place.