Some people don't want to sit next to:
1. People of the opposite sex
2. Children
3. Adults
4. Windows

And so on. The underlying reason is not relevant. The problem is that by the time you toss them all into the mix, the prospect of rearranging an entire cabin to suit everybody's preference is either impossible or hard and certainly causes delay.

Carriers are generally private entities. Other than government-mandated rules, e.g. for the disabled, if you choose a carrier, you live by its rules. If you don't agree with the rules, you either fly another carrier or you work to change the rules.