I'll admit right away that I wasn't a huge David fan. Can't say much else because it has been a long time time since I read that story arc. I'll reread those and come back here once I do.
Will say though that David wouldn't really be around very long as a rat. There are, yes, predators and lack of food. But the longest a rat can hope to live is 3 years. 4 years max if they are well off, and that's referring to pets. (I had 4 pet rats at one point)
So, maybe that was quite the punishment after all. Possibly worse than just being killed. Think about it: trapped in a morph that you know is short lived and very low in the food chain. They could have trapped him in...I don't know...tortoise morph! Those things live 100 years.