Ah ok, I misunderstood - yeah it baffles me why manufacturers don't do this. I make it a point to fuse absolutely everything at work, it's a minor expense to save a ton of time later.
Last week I blew up a $100 CO2 sensor because I wired it in backwards. Entirely my fault for not checking the pinout, but a 10 cent diode or a fuse would have saved me.
This is the same issue with telescope controllers. Plenty are rated for 6V absolute maximum, whereas typical power supplies in astro, e.g. the power tanks, or LiPos are 9V or 12V. Boom. Another $100 when a zener would have saved the day.
A zener costs atleast 2 cents a piece when ordered in bulk. That's 2 ENTIRE CENTS LESS PROFIT!1!
This was my interpretation, the host sets the output voltage to 20V incorrectly (due to a software bug or a hardware failure). Putting a crowbar circuit defensively on the peripheral side would save the peripheral.