If the federal government designed their own icon set it would be available for anyone to use without license restrictions. Since they’ve decided to use someone else’s icon system, they aren’t making a public domain set of icons.
But also, since they’ve decided to use someone else’s icon system, taxpayers are not paying for yet another icon system, when an existing one under an Apache license is available.
This is true. In some ways, Apache/MIT licensing is superior to public domain licensing as some jurisdictions (Germany, IIRC) don’t recognize public domain as a status.