If I'm honest, if I saw my child being told off by another adult, my first, childish instinct would be to bristle a bit inside - but this would quickly be replaced by mortification that I wasn't watching him properly, concern for the child who had been hurt and anger with my own child for being naughty when I know he knows better.
So I think those who are being precious about their child being reprimanded by another adult - swallow that initial feeling of affront that another adult is disciplining your child. You have brought it upon yourself by failing to supervise them properly. If you don't like other parents telling your child off for bad behaviour, make sure you are on hand to do it yourself, should the need arise.
If either of my two deliberately hurt another child they would be told off, made to apologise, I would apologise on their behalf and then we would leave. If they hurt another child accidentally they would be warned to be more careful and made to apologise, but we wouldn't leave.
But then I am a bit of a fascist about soft play, I have had my toddlers hurt by bigger, unsupervised children too often. I DO tell other people's children off if they're not vigilant enough to do it themselves. If they want to reserve the right to let their children behave like savages, they can keep them at home.