Hmmmm...I think it depends. For example, my dogs ride loose in the car in the back seat. That puts them at risk if we're ever in an accident, but I'm unable to have them ride any other way, so I take the risk. I wouldn't appreciate being told over and over again that it isn't the safest way to ride in a car, when I KNOW, I just think that is what works best for us. But if I had never thought about the risks of them being loose in the backseat, I wouldn't mind someone bringing it to my attention.
So is their decision based on ignorance, stupidity, or an assessment of risk vs. benefit? If the first, I'd likely say something. You unfortunately can't change stupid. And if someone says, "Yes, I understand the risk involved, but I have made a choice to do this for XYZ reason", then I'd likely shut up and move on.
So...I guess mentioning something in a polite way, then moving on if they're anything but ignorant?