As a retired AF senior NCO, I would have to guess that Heavy Metal music is allowed our troops because it would be unconstitutional to deny them the music that they want to hear, and also because it is tough telling a serving person who you have sent out to fight for God and Country that s/he can't listen to any music that s/he wants to hear.

Of course that could also be phrased, "Do you want to tell a suited up infantry troop with a weapon capable of firing bullets more than a mile in any direction that s/he can't listen to whatever music s/he wants"? We're dealing with 21st century troops here, so we have to allow them some freedon of choice.

Dave