I think the best defense of this rule is that (1) visiting the optometrist for time-to-time has early-prevention benefits that outweigh the exam costs and (2) r

I think the best defense of this rule is that (1) visiting the optometrist for time-to-time has early-prevention benefits that outweigh the exam costs and (2) requiring a new prescription when people get glasses/contacts forces people to have this exam. Not exactly honest, but plausibly net good.