Hey,
An enemy no question- Saruman deserved punishment. But (to me), we are thinking as humans in this regard. In our world, there is punishment, justice, etc. But I hesitate to lay that on the ent universe. I think they were there to Stop something, not to Punish. In fact, I will go out on a limb (hehe), and say that, an ent would naturally (or by its nature) not harm any creature, other than an orc, or possibly as self defense.
But to your question: Because captivitiy of a creature is such an affront to nature, I believe that for an ent, it would be a totally alien concept. Much like Bombadil, ents as wardens or even gatekeepers would really make lousy employees.
if that makes sense at all.....
