Don't most dog parks specify that you
may not bring unaltered males, nor in-heat females to the park? Like most rules, they exist for good reasons
Unaltered dogs are the responsibility of their respective owners. And each has the responsibility to ensure that "accidents" do not occur. LOL - it takes two to tango, and you can't just pass the responsibility in entirety to someone else (this is like birth control - it is the responsibility of BOTH parties to ensure it is used). If you're the owner of an intact bitch, that certainly means keeping her at home and on leash whilst in heat. But the responsibility of the owner of an intact dog to ensure that they are kept under control at all times (which means on leash in public) is just as great. Blaming some other irresponsible fool for bringing their intact bitch out doesn't alter the fact that
your intact dog was loose (ie. not under control) and able to mate, if that's what occurs.