First thing I'd like to say is that a farmer's market is not the best place to socialize Indigo, especially since you know that there are idiot dog owners there. I bet half of them think badly of you for having a "dangerous" dog at the farmers market, which is a place to go buy produce, not start arguements over dogs. If you really want to socialize her at the farmer's market, bring her in a muzzle. You'll have explaining to do, but the more people that ask, the more people you get to educate about socialization.

Second, and Joe Schmo can go out and buy whatever dog they want. Assume everybody is ignorant and idiotic with their dog and do what you need to do to keep the situation under control without expecting any input from the other owner. I know it sounds unfair that you have to go out of your way while others do nothing, but if you care about your dog, you'll walk a wide circle around a dog who is being walked on a retractable leash by a child. If another owner takes innitiative with their dog, great, but you should always have a handle on your dog first.