I seriously don't get how it's against some "social contract" or how it's rude to put your only carry-on, which is smaller than everyone else's, in the overhead bin. In fact, why is it my problem that someone is not willing to pay to check their bag and has to bring the entirety of their luggage on board? Why should I suffer because they are a cheapskate? Let me be clear here. EVERYONE that checks a bag is paying to check a bag. Some people pay by way of spending a lot of money on United every year and getting status. Some people pay by getting a co-branded credit card that has an annual fee. And some people pay at the check-in counter. But we all pay to check our bag in one way or another.
So if I'm the person who spends $10,000 a year flying United, or if I'm the person who paid $35 to check my bag at the counter, why does the person who decided to slow the boarding process and bring the entirety of their household possessions on board in some massive "carry-on bag" get priority? Why are they not the jerk who is violating the social contract? In fact, they ought to be grateful that if there is not bin space for them (meaning that they were probably in a late boarding group, meaning they probably booked a cheap ticket on Priceline and United is losing money on them), United will gate check their bag for free and they can still skip out on paying for luggage like the rest of us do.
I can't imagine any other business working this way. "Sorry Mr. Smith, we appreciate the $200,000 you spend every year at our hotels, but we need you to move your little sports car from the shared parking spaces and park it up against the side of the building so that the family who booked a $99 room on Priceline can get their minivan in".