There's actually research that making people pay for noncompliance (either upfront or post-factum) leads to less compliance, because people that can afford it treat it as a service. And giving that these events are visited by literally billionaires and a lot of affluent SV tech folks, making it a pay service would bury the volunteers under the mountain of trash. If the rules are "you MUST clean up", you get some trash slipping by. But if the rules are changed to "you clean up, or you pay a small fee and don't worry about it" - the amount of things to clean up would raise exponentially, unless you make the deposit so high the vast majority of people can't afford it - which would kill the event completely.