I agree, up to a point, that there's no need for us to criticize or try to correct other people's parenting methods or styles, that we should all do our best to accept and support eachother as much as possible.
Unfortunately, the way other people parent their children does often have obvious effects me and my children. It effects my childrens' perception of adults, of parent-child relationships, and it effects how those other children treat me and my children.
I quit working at a childcare center in a local gym because, as much as I enjoyed the employee membership and discounts, I was tired of my children being exposed to violent, threatening, mean parents; parents who openly hit (spanked) or threatened their kids with that type of violence, in front of my children, or who constantly criticized, ridiculed and humiliated their children, etc. I want my kids to trust adults, and I don't want to have to explain to my four year old why that "grown up" thinks it's ok to hit their kid's bottom, bite their kid, or scream at their kid, call their child names, or say mean things to their kids.
(Just for the record, I find parents who condescend, patronize, pamper, and compulsively praise and/or reward to be almost equally offensive.)
I might not tell you how to parent your kid, but I will ask you not to behave like that in front of me or my children, and I may tell you I think you're behaving very poorly, probably embarrassingly, not to mention ineffectively. I feel like the least I can do for my children is let them know that I will always do my best to stand up for them, and for other vulnerable people. You know, like other kids.