The research is publicly funded by US tax payers, but releasing it without any restrictions or delays makes it public for everyone, even for those countries who had nothing to do with funding it and are sworn enemies of the US and it's allies.
It does not cost $1k a year to have access to these publications as an individual, anything our "enemies" would want could be obtained for effectively zero dollars.
Generally speaking, science (and particularly publicly funded science) is an internationally collaborative process. Anything that would be of material harm in the hands of a rival government should either be classified or kept well protected within the bowels of private industry.
Is there any reason the believe that science publisher's gatekeeping has ever been a significant roadblock to nation-states that are already capable of industrial espionage and intelligence activity?
So, other countries and entities can fund weapons, armies, spies, but a paywall is a bridge too far to cross? That’s an extremely weak argument.