This should be as plain and simple as stating that "if there exists a fully functional open-source technology that satisfies a public institution's need, the institution should be forced to use such technology instead of any proprietary alternative, or to demand it's contractors to develop a custom open-source solution, even if it's more expensive then a proprietary alternative" (with some "sane" limit like it can be "up to but not more than 5x as expensive" - there's befits to society from funding "a bit" of os dev from public funds, but not over do it...).
"Software capitalism" and public infrastructure don't mix well. And private companies can still make tons of money developing, customizing/extending and supporting open-source software for use by gov clients.
But if it's paid from people's taxes, it should only be open-source!