Without first hand knowledge, I'll take a guess: if the owner of the vehicle isn't available or cooperating, then getting access to the USB drive requires opening up the vehicle without a key, which could require some time or specialized tools, or cooperation from the manufacturer.
But in general, this isn't really much new. If you have home cameras, and police can obtain a search warrant, then you can be compelled to provide the video, or they can seize any device (such as entire computer, your server closet, etc) that could reasonably contain the video.