I definitely see your point if they bring the dog IN the park. I suggested walking the fence line with the dog, and assessing her behavior towards the other dogs.

But yes, just walking into the park not even knowing how she looks/reacts to other dogs, big mistake. I have a dangerously aggressive yellow Lab, and it's hard to even walk the fence line with her (I did this with the trainer, and gave treats/praised her for paying attention to ME and not reacting to the other dogs on the other side of the fence). I can't take Rita anywhere, which is really upsetting to me as she is a very people-friendly dog, loves to swim, run, and interact with people. Even when I walk her on a leash with her gentle leader around the neighborhood, I've had people let their dogs off their leashes or let them out of the house (we always avoid other dogs, crossing streets/turning around when we see other dogs with their owners) and let them run up to her thinking she is friendly and will play. I have to try really hard not to go crazy on these people, because not only is this not good for my dog, their dog could get injured, and would it be my fault? Nope, my dog was on a leash, yours was not!