Do you mean how will it affect the NPS' receipts?  It will likely diminish some small percentage.  BTW: The parks are already free to active military. The idea of national parks is to preserve historical or natural sites for the public.  Can you imagine Yosemite valley if the property was held privately by numerous owners?  Billboards on Half Dome?  Yosemite falls power plant?  National forests and other land held federally and administered by BLM is another story and much of the land could properly be divested by the government into private hands.  Not a trivial problem, however.