I think some (not all) BMC drinkers see craft beer as a topic they have relatively little expertise in and thus find the idea of getting into a debate about it unappealing. They don't want their own inexperience highlighted and they don't want their preferred beverages to be insulted. Much like many craft beer drinkers incorporate beer into their own identity, I assume BMC drinkers do as well. It's a threat, basically. If I define myself partially by the BMC I drink, I don't find the idea of comparing that drink to other beers I am ignorant of appealing. Especially if I've been told time and time again that what I'm drinking is "piss" or "swill" or what have you by people who identify with craft beer. I think that's another big reason for the nuclear reaction to craft beer. Craft beer people are mean to BMC people sometimes. If you've been insulted for drinking Bud Light by "some snobby bearded hipster sipping out of a snifter" before, it's easy to get defensive. That "easy-going bearded hipster who just wants to share his latest IPA find" may get the response intended for every snob the BMC drinker has met. Best case scenario is for everyone to play nice and stop sneering at people.