To be fair BMI is not perfect. All your gym rats get lumped in Obese for example.
Not really. To get to obese by gaining muscle mass one would most certainly have to use certain illegal drugs.
Someone cared to downvote. So I made a random Google search and came up with this guy, who is 5'8 and 210 pounds in contest state
http://www.bodybuilding.com/author/john-hansen
Which puts him at 31 BMI, so just barely in the obese category.
And this guy is clearly not your average "gym rat", he's a winner of Natural Mr. Universe, so he'll be somewhere on the very edge of the Bell curve. 99.9% gym rats will never be close in terms of lean body mass. So if they are "lumped in obese", then they truly have a lot of fat to lose.
There are also many skinny people who eat nothing but dorritos and are skinnyweak. They don't make the Obese charts but they are no more healthy.
The fact of the matter is that people need to own their health.