Are PTZOptics cameras backwards compatible with USB 2.0?

Yes they are. 

The PTZOptics cameras should be able to pull in a 1280x720 signal from a USB 2.0 signal. 

There are some times this does not work though. If you are trying to go through USB transmitters/receivers over Cat cable, there are only 1 that we recommend - the Icron Ranger USB 2.0 Transmitter/Receiver Combo

We have tested other over Cat solutions and they do not reliably display the USB 2.0 video signal form the PTZOptics cameras. Issues can also arise using the supplied USB 3.0 cable into a USB 2.0 port. If you use an actual USB 2.0 cable, the camera should display its USB 2.0 video signal more reliably.