I've had people take pictures of my baby before. But they usually have asked. And my baby really loves the attention.
But I guess it would be creepy to have someone take not just one picture, but a lot of them. Sometimes people are shy or embarrassed to ask someone for permission first. Usually I would expect people to just snap one and then turn away. I guess take it as a compliment and keep a closer eye on your baby. It doesn't hurt to be overly cautious anyway.