It's cuz "most people" don't really look like the people on T-Nation. Most (many?) of us have a high BMI because of years of hard work, but we are a minority. As a dumbass means of easily checking if one is "overweight" or not, it does probably work most of the time (but none of the time for us). ...Eh, I still think they should change it.
I can't recall if it was on this site or another where a poster said his doctor walked in and said something like, "This is the part where I'm supposed to tell you and your washboard abs that your BMI is too high and you're fat..."