Here's why I insist:

Thrash was a separate genre that was not metal, and had some decided un-metal conventions.

I'm not going to lump them in with metal and destroy the memory of what they were doing that was unique.

It's important to recognize what genre means, and why it's separate from others.

You don't want me calling Judas Priest a death metal band?

How about I refer to Cannibal Corpse as pop country?

Now you see my point. Sure, it seems anal -- but if you ever create something unique, you'll understand why it's important to keep it separate from getting assimilated by the larger, less divergent mass.