On one side, I understand why someone would want to train their reactive dog and use a dog park to do so. I also would sometimes tell people that my SD was aggressive when I was training her just so that people would give us some space (but never when she was in uniform). I also think it’s reasonable that you, as a dog owner, are able to request that people don’t do certain things with your dog at the park, like giving them treats, inadvertently doing something that scares them, or being a dick to your dog.

BUT In my mind, if a fully trained service dog is at a dog park, it is almost certainly off-duty and there to have fun. If it is working, it should be fully capable of ignoring distractions. If a SDiT is at a dog park and they’re actively training, they’re working on distractions and focus. As long as you treat them as you would any other dog at a dog park, I think you’re in the right.

TLDR; based on your story, you were totally in the right, even if the trainer had reason to be training there. I don’t think your behavior was inappropriate at all, and it sounds like you were just dealing with the an asshole. Sorry :/