I think the main thing might be that someone placing a cache is installing a permanent fixture in the park, not just visiting and then leaving, taking all their things with them. People coming to look for the existing geocache aren't altering anything more than someone playing frisbee or soccer, but the placement of a geocache where it isn't wanted would, most likely, be seen as nothing more than littering to someone who doesn't understand the game and what it's about and what the cache is for.
I received another phone call from the Salem Parks folks today, from someone a bit higher up the food chain, and he said that in order to place a cache, I need to contact the guy in charge of events in the parks, fill out a permit, and pay a $15 fee. So it sounds like they're making it official. Maybe if this even they want to have gets off the ground, local geocache folks can speak with TPTB about that sort of thing.
It sounds as if they're making it kind of an arbitrary designation, instead of figuring out what geocaching really ought to fall under, because geocaching isn't an "event" any more than "walking through the park" is an event, really. Now, if a bunch of geocachers got together, reserved a picnic shelter, spent all day in the park, played games, hunted for caches, ate cupcakes and cheetos, and put up signs- that's an "event".
Seems like they need to hash this out a bit more, but I don't feel like I have the authority to speak to them about it, seeing as I'm awfully new to this whole thing.