I think its more likely that the people do know they just don't care and it helps them to put their backpack overhead so they do it anyways. There is minimal/no enforcement.

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floren 3 hours ago | parent [-]

I'm very much a we-live-in-a-society, follow the rules kind of guy, but if I checked a bag and only have my backpack in the cabin, you bet your ass I'm going to try and find a place for it in the overhead instead of cluttering up where I want to put my feet. The flight attendants can go scold the passenger with the oversized roller + backpack + 20 liter "purse" instead.

	▲	s0rce 20 minutes ago \| parent [-]
		Yes, the logical rule would be 1 bag in the overhead per person. If they enforced carry-on sizes strictly and charged less for checked luggage the problem would probably go away.