Delta is probably tired of crap falling out of the car seat and having to clean up the mess, or having the padding fall off and trying to figure which car seat it belongs to as well as dealing with the complaints. Similarly with packs and lose straps that tend to catch on things and clog up the conveyer systems which then leads to melt downs when no one's bags make to the plane on time. Then there are multiple complaints.
Oh yeah - just got back from Europe with my pack. Though just slightly larger than the size wise it fit in the overhead bin just fine - never checked it. And overall still smaller than the *&!$ roll-a-boards and oversized "computer" bags that most people schlep.