There are so many great points made in this forum........and I can agree with all of them. There really are many sides to every story, and I have certainly been guilty of jumping to conclusions when it comes to other dog owners. I have tried very hard in the past months to try and just worry about myself and my dog and hope that in due time any other owners who may not be be as schooled in 'dog park manners' will one day learn.
I live downtown San Francisco in the middle of an urban jungle, and have to take my own Lola to a park that is not designated as off leash....she can be very leash aggressive, even after we took the 'growly dog class', and unfortunately many dogs are off leash at the park...I simply explain to owners that Lola and I would be much more comfortable if they would call their dog to them if it begins to aproach Lola. We also always stay on the periphery of the park instead of walking right into the center where the bulk of dogs are playing.
I have also had an experience with an extremely aggresive boxer at our very favorite off leash park....I simply chalked that one up to the very characteristic female/female aggression that can happen with Boxers. Thankfully Lola was not the aggressor and she ran behind me when I pulled the other dog off of her. The other owner was very apologetic and removed her dog right away.
I can understand your anger, but sometimes I find that it's just easier for Lola and I to remove ourselves and walk to another park, (even if we are not in the wrong) instead of trying to change someone else who we don't know and who likely doesn't care about what we think.