It's worse. It's programming.It takes advantage of the human tendency to laugh when others do, regardless, (people will, even if they don't know what's funny).
Once people have been programmed to laugh, they equate the stimulus with humor, or at least de-sensitized to the normal reaction of disgust they might have had before. It's almost Pavlovian.
Given even a second to think about it, they would likely not laugh at all.
Ask Thomas Nast.