Feeding in the enclosure does not cause aggression. However, if the only time the enclosure gets opened up is feeding time, it can definitely teach the snake that opening the enclosure means DINNER! Once a person has trained his/her snake that, if he/she is stupid enough to put something other than dinner in there, like a hand... well, then, yes, a bite is likely. Or the snake will come lunging out of the enclosure wondering where the food is, which looks pretty darn aggressive. Another factor is that people who neglect their snakes by not opening the enclosure regularly to do things like clean the cage, clean the water bowl, etc, are also likely the same ones who neglect their snakes by not feeding as much as would be ideal. So that adds up to snakes that are half starved AND think food is coming every time the enclosure is opened. So it is easy to see how this myth got started. But mistaking a human for food is not the same as aggression. It can happen with the tamest gentlest snake ever if the smell of rodent is in the air, or on the hand. As long as you handle your snake regularly and don't do stupid things like trying to handle them just after holding your pet rat (or his soon-to-be dinner), they won't get overly aggressive nor be confused about whether you are edible or not.