The sub-genre itself is flooded with Satanic themes, granted. It's still not necessarily "the Devil's music". There are some death metal and black metal groups with Christian lyrics (rare, I know, but they're growing!).
This is why I say it has nothing to do with the music, itself. It has to do with what the artist(s) call attention to. It's the same as slapping a cliche and stereotypical content labels on anything else.
The only exception is country music; it
all deals in bbq stains, runaway renegades, and love and loss. . . and it all sucks. lol.