Looks like your printer does not correctly parse the HID report descriptor that Photon provides. Due to the fact that we are limited in the number of endpoints available, mouse and keyboard data is transmitted over a single USB endpoint (i.e. the Photon does not report itself as having two separate HID interfaces (keyboard and mouse) and instead reports a single compound HID interface: a mouse, keyboard and digitizer), however if your printer is USB HID-compliant, it should be able to understand that format.
There are some plans to support custom user-configurable HID devices (https://github.com/spark/firmware/issues/1294), but that’ll probably not happen in the nearest future.
If you’re OK with building locally and modifying the system firmware, here is a patch that shows a basic idea of what changes are required to report only the keyboard data (untested): https://gist.github.com/avtolstoy/8a5c0f78693cc6d1843d19040adaff52