Yes certain land is dedicated for that. The federal department is called BLM they have land for multi use, catle grazzing, farming, oil experation, etc. State might have some too depends on where you live. These groups who are picking and dropping people off are not advertising or hanging signs in the park for business and driving on a public highway that is used by multi uses. They are using all public access and not physical buildings that are for a purpose. I can see if they cannot go into the campgrounds or in the buildings to get people but the roads and parking lots? Come on lets be smart and keep drunk people off the road and use a bus driver that is able to get them to the park safely.
Again if their is a pay issue then that is why I am for a user fee rather then a general tax that all have to pay.