This is slightly tough to answer. The easy answer is yes. Of course you should expect the owner to control their dog at all times in public. Now comes the tricky part, What appears to be aggression to you (because your dog is scared) may appear to be harmless play to the owner of the offending dog. Some dogs are noisier and more active/assertive when they play than others. My answer varies depending on the circumstances. Ginger is a very timid dog, so I have to resist the urge to tell some to call off their dog every time she tries to hide. Ginger has to learn that at a dog park she needs to be tolerant of other dogs behavior. So instead of calling off the other dog, I will redirect Ginger's attention, and the other dog either loses interest or starts playing with her in a way that is more acceptable to both dogs. If the dog is truly aggressive, then they should leave the park, if they do not, I would have no problem in removing my dog from the dog park and alerting the grounds people that there may be need of a clean up in the dog pen shortly (and knowing me, I would probably make some rather rude comments on the way out ).