This is simply the best rule you can use. It works excellently for most municipalities and the skateboarding community. It is simple, supports the degree of personal responsibility and dignity that so many others lack, and puts the onus of improving helmet usage on the institutions and organizations that are wholly committed to the task.
Some states, regions, and cities can embrace this on an institutional level and some cannot. In many areas, this is the only rule that is unique to the skatepark. In every other policy sense, the skating facility is treated just like any other athletic attraction. Batting helmets are not required on the baseball diamonds, and the tennis courts don’t post a “no profanity” sign, and the skatepark receives the same essential treatment.
In those places where helmet requirements are already required by ordinance, it is usually a strenuous political maneuver to remove them. Few administrators, much less elected officials, want to be the “champion” of helmetless skateboarding, so when the discussion between skatepark advocates and Parks staff inevitably touches on helmet policy, if it’s required, it’s required.
How it’s enforced is a different matter. Local law enforcement can enforce that rule (or any other skatepark rule), or not at their discretion, particularly with encouragement or discouragement by City Council. They make such judgments all the time, every day. Many communities that are home to skateparks with mandatory helmet policies simply don’t enforce the rules, and skateboarders have showed up to the park—some with helmets, but most without—and the skatepark has enjoyed healthy, positive activity for years without any problems.