Of no great surprise to most I think, of all the nuisance fees airlines impose none irk passengers more than baggage fees. Unlike cancellation fees, seat reservation fees, etc, people believe they have no choice but to pay these (they could of course pack and travel lightly — read that TG article here). Airlines collected a staggering $27 billion in fees last year, a large portion of which were baggage fees, so don’t expect them to disappear anytime soon.
Here’s the link to this story.
I think you’re right that unavoidability is a big factor in the hatred of bag fees. As you point out, not everyone cancels or bothers to pay a seat fee, and many people carry their own food.
The other factor is the sheer size of the fee. A couple with two suitcases, round-trip, have added $100 to their cost (regardless of distance). If they decide to take a bigger case and share it, they save $50, but are now condemned to drag an oversize bag as they go.
No wonder some of the loyalty credit card issuers are using that as a selling point (“When I take my sister on a trip for her graduation, this card with 1st bag free, will save us hundreds of dollars”)