This is a thing? People sitting in wrong seats intentionally? Full disclosure I fly very seldom.
All. The. Time. It's actually crazy how many people assume they can just switch seats by sitting in the seat first then ask. Most common are couples, particularly newlyweds.
My system is this:
If you sit in my seat before I get there and then ask me. I always say no out of principle. You can't just sit in my seat and expect it.
If you ask me once I've arrived at my seat or after I've sat down, I'll only switch for a comparable or better seat.
Someone had the nerve one time to ask me to switch out of my first class seat into an economy seat because it was their honeymoon. Yea go **** yourself lol. You switch with whoever your wife is sitting next to in Economy.