Well, you wouldn't argue that a fighter had to attack with a reduced BAB for being feeble minded, would you? He is also using his skill to strike better, but just in a different way to a thief's sneak attack. Perhaps a lot of their fighting skills have become instinctive through combat experience and practice?
Looking at the RAW, the stats that drop have no effect on combat (except perhaps expertise when you lose the int prerequisite), so I don't think making feeblemind prevent sneak attacks can really be justified.