if you do nothing but just open the cage to feed your snake and clean the cage, they can get "trained" into having a feeding response when the cage is opened, it doesn't have to be underfed, its just all they know. breaking the auto-feed response once its there can be hard tho. Friends of mine have a few snakes that just have an auto-feed response, it almost look like he snake is chasing them until it realizes "oh, wait, thats not food", those snakes arn't handled at all unless I come over lol. watching some BHB videos it looks like they have some snakes that are like that also but, I don't know for sure.
but like Kaorte said, most of us feed in their enclosure and have no problems.