Q: How is “in the best interest of the United States” defined?
There is no real case law that defines "in the best interest of the United States” because it is a decision that is left to the unfettered discretion of DOJ—and that decision is not reviewable by any court. In practical terms, this means it can be defined as whatever DOJ wants it to mean. The governing statutes and regulations are 28 USC 516, 517 and 28 CFR 50.15.