The vandalism argument is something I hadn't considered. That's a valid point. The NPS budget was $3 billion last year, which represents 0.1% of the whole Federal government. But it's something that carries more impact than its budget would suggest. Of all the things to fight over, something that costs each taxpayer something like $20 annually. If it was up to me I'd spin it off, increase the price of our annual pass that $20 and run it quasi (or maybe even fully) independent of the Federal government. Maybe let the state park departments where the parks are located run them, they'd be closer in touch with local issues anyway.
"Silent enim leges inter arma." -- Cicero