"...such as the woman who pulled up next to my wife (who, true, was riding her bike sans helmet) at a stop sign, lowered her window, and said: “Did you forget your helmet or are you just stupid?”
(She was so flabbergasted and gobsmacked she couldn’t think of a comeback.)

She was gobsmacked because she didn't realize that not wearing a helmet is actually stupid? Is the wife one of those people who thinks head injury can't happen to her? Probably doesn't know or has never met anyone who has suffered concussion or brain injury?
Or perhaps she was shocked that someone else might have an opinion what she does to her own head.
If the wife was paying all the costs associated with a *preventable* head trauma, then maybe it wouldn't be anyone else's business.  But the taxpayer pays, and so you shouldn't be surprised if occasionally one of those tax payers calls you out.