It literally is their job. One of Mozilla's roles is to give their opinion on proposed web standards. It's one of the factors that determines what actually becomes a standard. WebUSB is Chrome (and derivatives) only at the moment. You can not like where they landed, perfectly valid, but they were asked.

Well, the reason is in the links I provided, and the reasoning doesn't scream arrogance to me.

Personally, I think choice is great. Why be upset when you can download chromium (it is supported by pretty much any platform FF is) and use it to do all sorts of stuff with WebUSB, if you are into that?

Still, I would like to see FF disable these features by default and allow opt-in. I don't see a great reason to avoid implementing them behind some "wall" (other than to avoid an increase in a concealed attack surface).