I think all electronic equipment on a plane has to have an isolating switch in case of a short circuit, so you can turn it off completely rather than risk a fire.
Also, I'm guessing they want to be able to turn the transponder off when the aircraft is parked up on the runway to prevent fire risk, or undue drain on the electrical system (and potentially giving ATC a nightmare when they try to work out what aircraft might be getting ready to move on the airfield).
But as you say I think pretty much every electrical system on an aircraft can be isolated in order to either prevent an electrical fire from spreading/continuing*, or because from time to time you need to either be able to reset them completely (if it's not working properly a reset or being able to turn it off to prevent it stopping every other transponder in the area from being unable to work properly), or disable a system to allow you to work on it.
Any of those three reasons would require that it be able to be done relatively easily, without damaging the systems, and that the pilot/co-pilot and any engineering staff know how to do it.
*I think that lesson was learned the hard way with problems on "essential" electrical systems causing fires that couldn't be stopped in the air by turning off a specific circuit (but rather an entire number of systems).