I'm guessing the people referred to here are the ones with grossly-oversized carry on items and the type of people who bring two carry-ons which don't fit under the seats, then try to stuff them both into the bins for a few minutes before making a scene with a flight attendant who insists they gate-check one (which, IMO, shouldn't be a free service, to discourage this type of thing).

Don't know about US domestic, but world-wide both the bin sizes and the seats vary a lot. My travel suitcase usually fits in both, but sometimes it's just too big for one of them. Without standard sizes it's just going to happen.