As it is your right to get up and move.
On a full flight? Move where? Most of the flights I've been on (prior to 9/11, I've only flown once since, and the flights were very nearly full both ways) have been absolutely full, nowhere to move to.
Why would I? That's my seat. By your logic, if I was at a restaurant and someone at a nearby table began eating my meal, it's incumbent upon ME to move. You've got serious flaws in your reasoning.