Usually it is the user group supporting the park. They raise the money through donations or fundraisers. The user group tries to have someone present during the most used times. That someone spends a lot of time putting on a smile and educating. With time and effort and encouragement visitors learn that they all have a stake in seeing that the park is kept clean.
Good signs help.
Thus, every visitor is encouraged to speak up when someone misses a poop. As odd as it may seem people really may not know how to find the bags, how to use them, and how badly it can screw things up if people don't do it. Community peer pressure. It is especially effective in the smaller parks where there is no place for the careless and clueless to be completely unobserved. The user group takes responsibility for picking up after the clueless and the careless.