Surely they would require some fat in their diet having a little bit of fat on would help keep temps up too. Its true that our captive buddies don't have to actively hunt for food so therefore wouldn't need as much food but I really don't think they would be picky. If a hungry wild snake was on the hunt, anything warm enough and not too big for them, that walked past would be food. I don't imagine them stopping pre strike and thinking 'nah, I just don't feel like chicken tonight'. Plenty of instances of a snake eating a whole coop of chickens.
Its true we must control their diet as they are still wild enough to eat everything that comes by them because in the wild they simply wouldn't know when their next meal was coming. I for one can't say I have ever seen an 'obese' snake. Personally I enjoy the thought of the reptiles I am responsible for feeling safe, warm and well fed. If the day comes that they start to look a bit too chubby and them favourite jeans just don't do up, well jenny craig is going to get an unusual call. Haha.
Mammals, of edible size, without something clearly poisonous about them, eg a platypus, would be fair game IMHO. Birds, much the same.
