I'm in the east as well, so it's not just a western thing. I'm not sure the sitting down thing is a specific strap in thing. Seems to mainly a social thing: get off the lift, sit down in a group, text, chat, etc. Rinse and repeat ad nauseum. Like teenagers at a mall.
Yep, that pretty much sums it up. I think they only do it because they're slow at strapping in, so they feel they might as well.