The back row is standard for UM's.
1. Because it is close to the FA's
2. They are always last off the plane, because they need to be escorted by the FA's. Having them in the back helps since I have had more than a few try to get off with out the FA's. Yes they can sit anyware, but it becomes more difficult to keep an eye on them. The last row is designated for UM's like the first row is designated for dissabled person's.