Because they are not paying for the research with their taxes and their governments are not reciprocating the openness.
This way everyone benefits. There is no way of keeping the access embargoed to US citizens or tax payers. But that wasn't happening in the current model anyway.
Classified research inside the US remains classified of course.
The benefits outweigh the cost several times over.
Nobody is saying that the government can't fund private research (as it surely does in the context of security, defense, etc). But that if research is _published_, it ought to be open access rather than behind a paywall.