The seats are assigned. People have been standing in line for 15 minutes now. Why on earth would anyone want to stand there, when they could just sit and wait until the line clears?
I understand wanting to get off a plane ASAP, but boarding? You just end up sitting on the plane, waiting for everyone else to get on.
If you have it in a backpack you can put it under the seat in front of you unless you’re in a front seat or one next to an overwing emergency exit.
This is all officially allowed and I’ve used it plenty of times.