maybe this will help so the events i do are about volunteer's the gambler and my event are about cleaning the forest we have had the largest volunteer clean up in the nation so 4000 people clean the forest for free . what should we charge them for music after all that hard work .we already not me but the gambler founder charges 40 per motor vehicle to stay in camp .that's to pay for camp cost.
People should be thanked - I don't disagree - but there is a cost and a value for this. If you thank people with food, do you get the food free? .. and if so, is it really free (as in no one paid for it?) Is storing, cooking, moving it free? Do you get free portapotties (again, truly free?). Is power in these remote locations 'free'? Each of these things have an associated cost - as does playing DJ. Do your guests think the portapotties have a value? What about the food? When you make it free, you remove the value. Vendors that donate items are also donating a value - and quite a few of them can write that off (sadly, by established laws, a vendor cannot donate their time or services .. aka DJ). Unless specifically addressed, the fee your group charges .. is ultimately what people think pays for all of these things.
Example: Guests at a wedding usually attend for free. Was it truly free?
Also, if you have a large amount of people in an area (like 4,000), you can't play music just because. Someone has to pay for the rights to play that music (Royalties) - even at an all volunteer event. While I'm sure there are volunteers, there are other costs involved (several listed above). Performing - is another cost and it should be valued. Showing a movie to this same group will also require heavy duty equipment and royalties to the movie associations. Equipment for a 4,000 person event is not light, it is not quiet nor is it cheap. Everything has a cost.
A very big legal difference between a wedding and the the event you're discussing: A wedding is a private event among a small close group of family and friends. Your event is a public event - this alone changes all of the rules. Add in the resources and financials and it gets worse from there. The promoter(s) will be on the hook for any and all musical permissions and royalties. As relates to parks, many parks require permits for musical events (among other things) - especially if it entails groups as large as you're discussing.