Deep Purple and Black Sabbath are considered to be the originators of Heavy Metal rock music. Sure, metal has gone through changes over the decades, but you can't suddenly decide one of the two bands that originated the sound creating the rock sub-genre isn't heavy metal anymore.
And I'm in complete agreement with uncletravellingjim's categorization of Zeppelin (except that I can't stand their music at all, and consider Page just a flat out thief).