But rarely do those failsafes protect reliably against 'the mainboard was splashed with salt water'.
Even with triple redundant relays, how do you know the salt water didn't just wet them all?
LEDs are diodes (Light emitting diode). Certainly this was a cost saving measure, but it's not a bad assumption that the LED wouldn't allow reverse current flow.
People usually respond to this by saying that it would be absurd to suggest the company did this for its own benefit, when anyone who engineers knows these are often caused by revising design to minimize costs... and increase profits.