Huh? What part of their genetic make-up or DNA would tell them to stop eating?
They're opportunistic eaters. In the wild, there's tons of uncertainty as to when their next meal is. BPs don't have the ability to understand they are in captivity, being cared for by an "overly generous" owner who will offer a nice plump rat the next time, if they decline this meal.
There certainly are fat BPs out there - I see it all too often in breeders who are plumping up their females in order to get to breeding weight ASAP. Maybe I'm the only one out there, but I'm keeping my pied lean.