A very under appreciated un-written rule

I believe that the rule of not eating while taking a shit is one that is very well followed throughout the general public.  I do not ever recall a time when some one told me about this rule, but for as long as I can remember, I have always been a strict follower of it.

I wonder if this is instinctual or has something to do with social behavior.  I remember coming across candy rappers and disposed food items in the bathroom of my apartment from time to time and always being a little bit confused.  Did some one walk in as they finished the food, and throw out the garbage on their way in?  Or did some person eat the food while in the bathroom?More importantly did they eat while taking a shit?  

I would be very interested to find out more about the subject.  Perhaps speak with some one who does not follow this rule.