The policy you mentioned only allows people to carry a firearm if they are otherwise allowed to carry a gun in public according to state law. This means they cannot be a convicted felon or otherwise disqualified, and in most states they are also required to have a concealed carry permit: https://www.nps.gov/articles/firearms-in-national-parks.htm
Aside from natural events, humans are the biggest threat to other humans in parks, and now they can pack guns legally.
Unless you set up security checkpoints around the parks, how would banning guns in parks prevent people who intend to harm park rangers (which is a serious crime) from bringing in guns? I don't believe that a person who intends to commit murder will be deterred because committing murder requires them to break an additional weapons possession law that cannot be well-enforced given the nature of rural parks.
As sneak said in another comment, "People who would shoot at park rangers would not be deterred by such a prohibition, so the change does not make them any less safe. It does, however, make normal park-goers a lot safer."
In particular in the US there are more guns than people. Attempting to confiscate all those guns would inevitably fail and likely lead to massive civil unrest. It would also fail to prevent criminals from still having access to guns.
Perhaps an arms-race is unfortunate but unilateral disarmament on one side is not a practical solution to the problem.
FWIW the US is not the only country where citizens are allowed to have guns. Look at Switzerland for an example.
Relative to the USA status quo, Swiss gun laws are like a far left socialist wet dream. No electable left-wing politician would ever imagine being able to get one tenth of Swiss-style laws passed in the USA.
The debate is never “guns or no guns”. The government and its agents always have guns, even in places like Japan. The debate is whether or not the unwashed masses also are permitted the right to defend themselves alongside (and sometimes against) the agents of the state.
History shows us quite plainly the dangerous issues with the circumstance where only the state is armed and civilians are not, from the holocaust all the way down to the 2004 Republican National Convention in NYC.
Secondly, National forests are a very important component of public lands for hunting: I for one find it awesome that I can stock 1 year worth of organic, grass-fed meat from legally harvesting an Elk on state or national forest land [1].
Finally, hunters - with our firearms and ammunition purchases - actually supply the bulk of $$$ that go towards maintaining public lands and wildlife [2]. You are welcome!
1 - https://www.fs.fed.us/visit/know-before-you-go/hunting
2 - https://www.npr.org/2018/03/20/593001800/decline-in-hunters-...
It turned out he'd been a park ranger at a state park in a rougher, rural part of the state. He told me me that large groups of teenagers would hang out in the park in the summer. They'd get drunk and start these massive brawls with each other out in the woods, and my tow truck driver would be the only person around to respond, often in the early hours of the morning. Budget cuts meant he was the only one of course, and local police weren't going to bother driving way out to the park. He seemed like a genuinely compassionate person, but he said a lot of the time, he'd just have to let the fights happen because he was so outnumbered he couldn't break them up. He told me he only lasted a few months.