Quoted from Arcade:
Makes you wonder why the caps say "When lit" on them though. lol
So if you (under decently rare circumstances) trigger them while the motor is running you don't complain that you were "cheated" out of points, hence why they shut off while the motor is running, it's pretty standard on all Gottlieb games that have always-lit pop bumpers. Many games do actually do as assumed though and only activate the lit point score sometimes through various methods/challenges.
It's funny though because if it's a single (10, 100, 1000, etc.) point score it usually doesn't run through the motor anyways, but like if you strike it while you're being awarded several points of the same denomination you may not correctly get your point score meant to be given from hitting it. Insanely rare but still. Matters more on multi-point scores (50, 500, 5000, etc.) because the motor needs to run for that - and if you need that while the motor is already running, it obviously creates an issue. So while it didn't matter much for the single-point scores, it probably became standard and also convieniently covered the extreme rarity of the single-point issues as well.