If people would look at places like Wildlife Prairie State Park in Peoria, they would see that private people and corporations have been giving support for years. These people and corporations get their names put on a plaque that is hanging on the wall. It takes nothing away from the park, and they do not get to build a fast-food restaurant, store, gas station or office space on the property.
The funds are used to feed the wildlife and maintain the property. This could be, and would be a fine idea if put into effect for the national parks, also. The plaque just shows that people do give funds for support. It's not a billboard, just a small plaque in the gift shop.