Your example is not entirely appropriate because first you're comparing 2 classes of paying customers: the ones that paid a lot, and the ones that paid a little. Then you compare a service that's provided for all passengers equally, like the duty free.

My point was being overweight is free, being "overheight" or an invalid will cost you every time. I have to pay a lot more to get the same level of (dis)comfort that everyone else gets for free.

It's the kind of discrimination that people just let slide but it's discrimination nonetheless.