jjvan said it............... it is so we can better keep an eye on the little ones. Of course, it would all be easier if we could just put them in the overhead bins, then take them out after all the passengers are off.... of course, you do this once or twice, then there is always going to be a parent that complains......etc, etc....
But seriously..... you can only imagine the moment of panic when you are trying to scan the crowd of passengers in the aisle during deplaning... trying to search out those UM's (which you've reminded numerous times to remain seated until we come to get them) and you can't see them.
Putting them in the last row is a process which allows us to better ensure their "safekeeping" until we officially hand them off.
I've flown Lead numerous times where I had seats open in 1st. When available, I've moved them up there..... I love the look on their faces when they realize that they are going to sit in first class.... but this also allows me to keep an eye on them during deplaning.
So NO, we don't put them in the "back of the bus" as some sort of punishment, it really TRULY is for their well-being.