It's not about if the camera is a USB device or not, it's about what the capabilities being exposed is. Web browsers aren't just handing over the webcam as an arbitrary USB device, they are presenting a media stream from them. Which means they can't for example send arbitrary commands, flash firmware, or do any of the infinite things a USB device might present.

I don't care?

I'm not giving any old website access to all of my USB devices. I'd expect to give a website I trust access to a specific USB device. I'm not sure why you think this has to be a willy-nilly free-access-to-everything feature.

Yes, I want to give access to site XYZ to a product purchased from site XYZ to do amazing things over WebUSB.

If that isn't the current spec, then change the spec so it's good.

But Apple just doesn't give a single shit to propose any changes because doing so would hurt their precious app store just a little bit.