This reminds me of something related. I don't force my children to wear helmets when they ride their bikes. When we are out, people yell at me "Where's their helmet dad?". These are the same type of people who force the removal of swing sets at the parks in my area.
Personally I do not wear a bike helmet, however, if some of my former mountain bike friends called up and took me out riding all day on trails I did not know then I would find a helmet and make sure it was properly fitted to my head. So I am not absolutist when it comes to helmets.
As it happens I have sold thousands of helmets and fitted a few hundred for people. Along the way I read the instructions and you do need helmets to fit properly, i.e. with the straps tight. I have also noticed that most cyclists including those in the peleton ride along with the straps somewhat less tight than they should be. Hence I suggest that rather than have helmet debates with people you helpfully point out to them that (if they are wearing a helmet) that the straps are dangerously done up. Point out to them that they could get strangled by their helmet in an accident if, due to the straps not being very tight, the helmet slips back. Assist them in getting their helmet setup properly, by which time they should not be able to move their jaw and give you any more lip :-)