Because he has the potential to help them. They will presumably assess him in practices this week and determine if he's one of the 45 they need for this game. Just like every other player.
My guess is that they expect that he will be active and have some cautious optimism about him.
But even the most optimistic of coaches knows what we all know: he's been out of the game all season and was not particularly good last season, despite his impressive career resume.
For some reason, I can see big things out of this guy and am thinking he'll play and make some good runs. But him either being inactive this week or getting just a few plays would not surprise me.
The X factor in all of this is that Iosefa showed something last week. I assume they will measure what they expect to get out him versus Bolden or Iosefa and if they are not confident it will be more, the right move is to put him on the inactive list, no? Said differently, do they really need to carry 4 RBs?
Last, Jackson could be inactive this week while they hopefully take care of business and give him a long look during the extended garbage time against the Fins. It's possible he needs a little more time to get in football shape and learn the offense, and the combination of that and the prospect of a regular season exhibition game in Week 17 might keep him out this week.