When Motorhead started out, nothing was called 'heavy metal' or 'metal.' Back then it fell under the 'hard rock' label along with groups like Black Sabbath, Led Zeppelin, Heart etc. For those who remember when the term 'heavy metal' started getting used, those same 3 groups I mentioned were now called 'heavy metal'.
Anyway, some labels actually hurt the band once they are associated with them. Remember 'grunge,' 'alternative,' etc. Anyway labels are a form of marketing and most professional musicians/rock stars that I know, don't like being called by what people or the record companies have labeled them.
Labels are not only form of marketing, at least they were not before 80s. Rnr is not just some label for marketing.. Labels are different mentality too. I told about example Rolling Stones and Napalm Death. Genre's are not just some labels, and Motorhead and rnr is not just some marketing fun, its art. Maybe Britney or Rhianna are just fun, very bad one, and just labels... But about other stuff you wrote I agree. And those groups you said "they are now called heavy metal", well I dont call them like that and I think many companies, people's on this planet dont called them like that either. Those who called them like that are companies, people, magazines who are metal, or connected in some way on that genre.