The main thing is consistency. If we aren't going to allow races to get out of flying via one method, why allow them via another? That, and a bird can be a bitch to hit in the first place, why give it good aim as well? Of course, the ward rules aren't consistent(why only some houses are non-wardable and others not, eg: italy, estate), but still.

Now, athena's ultimate gets you out of flight as well, but that is not via your doing (and a flying crow goes SPLAT, without items).