You could say the same thing about the term "conspiracy theorist". It's just more convenient to label opposing voices with a single, derogatory connotated, term. Especially when the aforementioned group is comprised of many genuinely stupid/malicious individuals.

Now the problem with those groups is that their borders are made intentionally blurred. In the case of climate change: industrial/government lobbyists and shills try to mingle with genuinely skeptical citizens. Grouping all these people in a single group is one of the only ways to fight against that, especially when the raw information dissemination power of the opposing party is so much stronger.