I'm always annoyed when I wander into a store to shop for groceries or something and there's other people's kids running around making noise and all around being disturbing (grabbing my leg, yelling, rolling about on the floor and etc.). My parents would NEVER have let me behave like this in public!
However, whenever I respond as I tend to:
*cue best Uncle Scar impression* Where are your parents?
I'm considered to be rude and stepping out of line!
Why is it considered rude for me to take over when parents are obviously neglecting thier duty to society?
I don't think you are rude -- but some parents do take it pretty poorly. (Obviously if they're negligent enough to allow their kids to run around without respect for everyone else, they're probably also the sort to be offended if you insinuate they are "bad parents" by correcting their kids.)
We had a situation where a kid at church had been cutting up for a long time (he was obnoxious, disruptive, using foul language, bullying other kids, etc.) and the teachers were ineffective because the parents wouldnt' believe their child was behaving that way. Finally one of us took him to task for it because our kids were sick of it, and his parents showed up and ended up marching him to our house to apologize to our boys; but on their way out they insinuated they would have rather had us go to them [which of course wouldn't have worked, but whatever], and ever since then they've avoided us and do not talk to us.
(Which actually isn't a bad thing, but it's just rather crazy. If our children had been treating other people that way, we would have WANTED to know and would have changed what we were doing, to make it stop. They just were embarrassed that their pride had been hurt, and meanwhile their son's still acting like a jerk.)
Generally, if you can go through the parents, you should.
Then it seems almost more prudent to avoid the situation, to spare yourself some grief.
And if you discipline someone else's kid, expect to be looked at as the villain.
(And if I guess that, if I were in their shoes, and I didn't think my kids were being bad; and some parent publicly criticized my kids, I'd feel embarrassed and angry. So I guess I am saying, it's reasonable for you to expect a negative response.)