Well, I can sure see both sides of the issue, if I fill in details. Which you haven't really given, so this is only one possible interpretation.
It would have been wiser for the other mom to wait until you came back, UNLESS you had been letting your little angel get away with behavior that other parents or children found distressing, but couldn't figure out how to ask you politely to get the little guy under better control yourself.
You were there; we weren't. Is there a possibility you are underreacting to your son's behavior and overreacting to the other mom's response? It's a common pattern. I have frequently watched parents be amazingly indulgent about their kids' behavior while everyone around them was biting back incredulity and anger.
If this is not the case, then I would tell the other mom pretty much what you told us. You feel angry and upset because she reprimanded your son (and perhaps did so with inappropriate physical force?), and you would like the full story. Period. Without that information, you have no real basis for knowing whether her intentions, however poorly acted out, were good or not.
I tend to agree that misbehavior, whether in children or adults, is something that others should probably address. If a child in my presence is acting in a way that's causing distress to others, I am likely to speak to the child and ask for better behavior, though I wouldn't yank her unless it was to prevent danger to herself or others. Likewise, if I see a parent treating a child thoughtlessly or cruelly, I am very likely to say something.
I've had a number of parents mad at me over the years, but I can take that more easily than worrying that I should have spoken but was afraid to. We all have to live together on this planet. Why should we condone bad behavior with our silence? We all have to live with the consequences of turning away and pretending it's not our business. In that light, you may not wish to pretend you didn't see what happened if the other mom was truly out of line. And she may not have been able to pretend your little guy was being a good citizen.