As for why we didn't go back, his decision to try taking the wheels off was completely unexpected -- he had been dead set against it for quite a while -- and I didn't want to take the wind out of his sails by saying, "Okay, but you have to wait 15 more minutes while I quickly hop in the car, drive back home, get your helmet, and come back with it."
It was a calculated risk, and you can always argue that we shouldn't have taken it.
The point of the whole story is exactly what you said in the last sentence. It's not so much what people think they're entitled to say, it's how they say it. Just look at the replies to my story: you've got a whole bunch of people who think it's perfectly fine to lecture me about how I'm part of the problem here and to do so in a rather hostile manner.
This is the kind of thing I was trying to point out in the first place, the fact that the more and more people these days feel entitled to be crappy to other people in order to prove some point.
Yes, teaching bike safety is important, but it's also not that big a deal.
You shouldn't have had to endure a lecture or be threatened by your neighbor. If the kid is just learning to ride a bike without training wheels, it would have been worth it to go back and get his helmet. I hope the lesson isn't "you shouldn't tell people when they are doing something dangerous," but that you shouldn't be a complete prick about it.