My printer injects hum into my USB ports
This is probably a combination of the printer injecting noise back onto the USB power rail, and speakers letting some of that power noise get into the audio signal.  The audio data is sent over USB digitally, so there are no analog signals in the USB cable to inject this hum onto.

There is nothing in the USB spec that says power can't be drawn in pulses other than the maximum current.  This is therefore squarely the fault of the speakers.  The circuit in them is crap.  If you can return the speakers, do so.  Otherwise, live with the hum, live without the printer, or get speakers that were designed by someone that actually knew what they were doing.