Exactly. Most of the first couple of dozen posts here called it "public" or "first come, first served", but that suggests there's nothing wrong with putting multiple bags up, or using space in the front when sitting in the back. Whether or not this violates airline rules, it certainly violates courtesy.

I wouldn't ever remove someone else's bag, but I would re-orient a poorly stowed bag (e.g. to wheels first) if that meant I'd have room for my own, making sure they both fit and the door still closes.

While I advocate using space near your seat, so others don't have to stow bags way aft of their seat, specific spots are unassigned.

On most narrow-body planes, there is not enough space for even one rollerboard per person. Typically, there are 2 bins (6 wheels-first bags) for 3 rows of 3 seats (9 pax).