I don't know all the details, but as I understand it, you don't necessarily need a USB License ($5000/year membership fee) to have a microcontroller running USB-HID across Windows.
Though I haven't figured out all the details of how all the mechanics work, its still interesting to work through even the most "toy" example of the USB-HID stack.
You'd get assigned a VendorID if you have a USB license: That is a USB-IF thing, not a Microsoft thing. Then each licensee gets to assign its own ProductIDs.
There is no authentication in the core USB protocol: A device can declare any VendorID. Major microcontroller board manufacturers such as Sparkfun and PJRC provide specific VendorID/ProductIDs to be used by DIY projects using their boards ... but I've seen some of those being reused by other boards. I have also seen the VendorID 0xf055 used by free open source firmware.
I received one for a virtual usb keyboard [2] that I made to work alongside a desktop app. Having a dedicated VID/PID makes detecting the presence of the device easy.
I think if someone wanted to have a very simple interface to control I²C devices over USB, it looks like there's plenty of cheap chips around that expose an I²C bus as a simple USB serial port to your computer.