USB usage restrictions
Is it required by law (US or any other country) to have a VID/PID combination that is registered with the USB-IF?


It is required by the license agreement, which is a kind of contract. It becomes a matter of law only in the sense that most countries have laws that cover contracts.


  If no, why would you still want to register one?


Because you want your device to be recognized as being distinct from all of the other devices that are already out there. And all of the other device manufacturers want that, too.


  Is the trident logo free for usage without a Logo License Agreement (this logo does not appear in the logo usage guidelines)?


No. If you put the logo on a non-compliant device, this will cause harm to the USB community at large. They will come after you in order to protect themselves.