The problem is that the USB standard makes a basic assumption:
the host computer is the one with the beefy power supply and all client-side devices don't
For the Pi + Arduino/RAMPS pairing, this assumption is just wrong. The downstream side is the one with the beefy power requirements and the Pi was designed to have little power.
So the Pi in some cases is expected to provide (some) 5V of power to the printer for the device stack to work and to recognize the connection. If the printer board is otherwise turned off then the Pi tries to sink power over that cable to the printer. What printer manufacturers should do is to consider the possibility of something plugged into the Type B port as accidentally back-powering their board, detect this and to provide circuitry to prevent it. They don't.
In some cases, both Pi and printer are powered and yet that 5V from the Pi is traveling over the serial cable and then to ground on the printer side. In circuit design, this is expected to happen but the printer board manufacturer should make sure that the resistance to ground is so high that the current is limited. But again, they don't.
I've just sent in a query to Adafruit to ask them to source or make a serial cable for this purpose with manual and/or programmable inline 5V switch.