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.
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