Why do americans put the metal genre under the "rock" tag?
Not all of us do, and in fact, I'm very glad to meet someone else who agrees with me that this is an important distinction.
Metal is its own genre, composed by its own standards. It emerged from rock (specifically, prog rock, soundtracks to horror films, loud hard rock and early punk combined) but it is not rock.
If you're up for some analytical historical data, would you read The Heavy Metal F.A.Q.?