Ummm....I'm a preschool teacher and I do it all the time. I have two philosophies on this:
First: It Takes a Village. I would *want* someone to stop my son from doing the exact same thing if their child were being hurt, and would apologize to the mother as well as encourage my son to check in with the offended party. Unfortunately, this feeling is not widely reciprocated in our culture. We tend to want everyone to watch out for the safety of our children, but to shut up when they do something wrong.
Second: If You Choose Not to Parent Your Child, I Will. As parents, we have to learn how to position ourselves so that we can see our children even when we are tending to a child. I've had multiple park experiences, where I don't even know who the attending parent is because the parent isn't watching their kid. On numerous occasions I've had to step in because children are being unsafe and I had no idea who their parents were. My perspective: if you are texting, or chatting with a friend, and you aren't tracking your kids, I'm under no obligation to figure out who you are and report to you when your kid is hurting others or being dangerous. My job is to make them stop. If they care enough to be upset with me (and some have), they need to get off their butt and parent their kid. I've got no problem with redirecting children or even making appropriate boundaries. ("You can play that way over there... this area is for the little kids"-- that sort of thing.) My position is simple-- if you don't like how I'm tending to your child, do it yourself.
I tend to think that the mother you referred to has some issues with seeing her child's negative behaviors, and that's likely where her anger (or is she embarrassed?) stems from. Without asking you, she assumed that her four year old had been wronged and immediately gave the child an "out". Unfortunately, there are parents who will "rescue" their child from unpleasant situations because they don't have healthy limitations and boundaries with their children. Instead, they either don't want their child to 'feel sad' at appropriate times (which suggests mom has her own emotional issues) or there are other issues at play. You don't have to be negligent to be dysfunctional. :) IMO, a four year old who has aware parents should have been instructed long ago not to hit, and certainly to be safe with babies, and those parents would have followed through with making amends, not angry with you.
In my experience, it's usually the people who aren't actively parenting or those who are dysfunctional with their parent/child relationships who are the most offended. Me, I'd be embarrassed and apologetic. But certainly not upset with you for taking care of your baby.
Just some extra food for thought: I grew up in Honolulu with a Filipino family (my adoptive dad is Filipino) and TONS of Aunties and cousins. I was raised to respect anyone older than you are, and to listen to them, because if I didn't and Mama found out, I would have been in SO much trouble! Mainland mothers are different, their children seem to be more in the "personal property" category than "community property".
Just my opinion.
Added: I have to disagree with all the people who say 'don't raise your voice'. I do use a stern louder voice, sparingly, (when my preschoolers are ignoring previous redirection, and *only* for safety issues) but it I don't think it does our children any good to pretend they aren't doing something egregious and play nice. It's not ideal to raise our voices to children, yet when they are choosing to be unsafe and not to stop-- here, I think it's okay to let them know you mean business. Used sparingly, a raised voice makes an impression.
H.