It happens with MBTI as it does with most social groups. People like to feel set apart from the unwashed masses in some way. This can be accomplished by believing one possesses special truth, has unique insight, is more skilled or intelligent, etc. I think it is human nature applied to MBTI.
I also think some people are unlikely to view MBTI as a group with the kinds of cohesion you see elsewhere. It does generate those tight social groups, but it has taken a long time for me to understand that. I have no sense of belonging as a community member with any MBTI site even when I post on my type site (outside of a few direct friends, rather than belonging in a social role). It's more like observation. Because it is a theory, it can seem detached. When applied, it can become socially based.
Edit: This seems like an important distinction because I have seen it become the core issue for various online controversies. Those members who have a strong sense of group dynamics do much better interacting with the core groups on forums. They can cross type forums better as well. Familiarity with church groups, small-medium sized towns, and any other tight-knit social groups that function on that level helps a person understand instinctively the kinds of faux pas and entitlements that go unspoken in group dynamics. People who do not have the experience or ability to embrace those dynamics can end up harshly judged because it is assumed they do understand it, but are directly violating and offending. Or else the outsider behavior becomes so incomprehensible and foreign, that it is assumed it must be negatively motivated. I have limited ability to function in social dynamics. Because of all of this, I have patience for someone who is socially "out of line", but I know it can just as easily be me due to social spaciness.