They're only objective on the broadest of levels. Iron Maiden is not rock, but they are metal, which in turn is a subset of rock music. Lady Gaga is very clearly not rock, she is pop. Enya is not jazz, she is new age. Those are facts. Once you go down the rabbit hole, things get more and more subjective. Wintersun on Metal-Archives is listed as "symphonic melodic death metal" - uh, okay then, you could also just go with "folk metal."
Overall, subjective terminology is subjective. But there is, at some level, objectivity to it - again, Katy Perry is pop, not classical. J.S. Bach is baroque era classical, obviously not rock music. The more you get into subgenres, the more annoying it becomes, the more subjective the terminology gets, because not everyone will agree, and sometimes it just gets to be way too much, at some point you start getting to labels that only apply to a handful of bands or even one artist.