Nothing gives. Somewhere along the way Mayhem made a black metal album, Morbid Angel made a death metal album and Helloween made a power metal album and all three musical movements, while still heavy metal, were so radically different that music journalists coined the term. Of course, you have plenty of idiots who mis-use the term and plenty of self-righteous *****s who make it their duty to police the usage of it.
Some sub-genres are so narrow and nebulous that its just plain dumb, I do agree. But even look at the 70's: Psychadelia, hard rock, heavy metal, progressive rock, rockabilly, folk rock, blues, southern rock, and jazz fusion makes up a Classic Rock radio setlist, among others. Its just that with the internet, journalistic musical terms have spread like a plague and every dumb kid with an identity crisis has to attach themself to something ridiculous. But believe me, sub-genres are journalistic in nature, when used properly.
(Im sorry but some people's little subgenres of 5 bands are just silly. Just throw in black metal at
www.metal-archives.com to see a list of 16,000 different black metal bands, for example.)