Don't sweat it. Parenting is an adventure in learning. I had an issue earlier this year where an adult started chastising my 10 year old son about something. At first I was caught off guard and didn't think the guy would keep on like he did. I waited for the people that were there running the organization to step in. Nothing happened and it went longer than it should have. I did step in and ended it making it clear he would never speak my son that way again or any child if I was around. Still I felt horrible for my little guy as he didn't deserve what happened.
I spoke to my son later and apologized for not coming to his defense earlier. I explained the situation to him and he was fine with me and told me it was okay. I told him I was proud of him for being respectful and taking it like a man. A lot of parenting is trial and error. I can totally see how this would upset you that it happened and that you didn't feel your response was fast enough or correct. I think the fact that you are aware it made you uncomfortable is the sign your on the right track of parenting. As @Cowcatcher said have a talk with your child and explain that its okay to politely tell strangers no thanks. While probably 99% of people are just overstepping and not a threat. They still don't have the right to touch your child.
