How much is too much?

When witnessing a parent being verbally abusive, at what point should you do something? I recently left a store with tears running down my cheeks. While in line, I heard this woman cursing and emotionally bashing her four year old daughter. I am the first one to admit that there have been occasional outbursts of frustration in my home but you seriously have to know when to lock yourself in the bathroom and have a good cry. In this case, sadly, I think what I saw was pretty much her daily life. What amazed me was how well the child was behaving in the beginning and how the mother didn't seem satisfied until they were both upset. As the little girl became unglued, the mother seemed to have more fuel feeding her fire. I almost said something several times during this episode but then wondered if the child would pay for it later. So, I was terribly upset and often recall the desperation in those big brown eyes. I hugged my little ones tight when I got home and wondered if I had done the right thing by not interfering. How much is too much? Is it wrong to tell strangers how to behave?