I've gotta say that it annoys me when people put their bags on a seat designed for a person. Sure, there were empty seats available. However, I've seen situations where a person sits down, puts their carryon bag on the adjacent empty seat, then gets involved in reading (or zones out or whatever) and never moves the bag when the abundant empty seats get occupied and an actual human needs the seat.

That's no excuse for deliberately sitting on someone's bag, even if the bag is occupying a seat. I would not do that, even though in the privacy of my own mind I might think a certain person is a jerk for taking up a seat with an inanimate object, perhaps forcing others to stand.

I might, politely, say something like "Would you be so kind as to let me sit in that seat, seeing as how seating is scarce right now?".