I've only encountered this a handful of times but it happens.
It is much worse in other countries. The aisle will be full before the plane has fully taxied off the runway.

What's the FA really going to do? If it was just one person, maybe they could get singled out for punishment but when its a group, they're not going to do anything. Plus, is the Pilot really going to block a busy taxiway (or runway) waiting for people to sit?

Actually there was one time we parked at the gate, seatbelt sign turned off and about 30sec later when everyone was in the aisle and bins were open, the pilot came on and said they have to move a few feet up. He said "I'm not going to make everyone sit down, just hold onto something".