I never do this (except for family members/close friends) but if someone else's child is bothering my or another child/doing something that needs intervention but the parent is momentarily unaware, I would usually try to find them first rather than get involved myself, wouldn't you?
I've just had a woman tell off DD for something when she was right behind me and it really pissed me off (for 5 mins). Yes DD was being irritating and needed telling off, and yes she wanted to protect her child but FFS I'd rather she asked, is this your child (obviously was as no one else was there) and let me deal with it rather than go charging in with a very over the top reaction. I usually laugh off little tiffs between toddlers but some people take them oh so seriously.
A bit like people rolling their eyes and shaking their heads if your toddler has a tantrum in a public place - FFS have some tolerance people! It's just what they do and I can't help the timing or the place when it happens!