Public funds are often used for private purposes. For example, government representatives who are members of groups that have no connection with government, can use taxpayer funds for transportation, per diem expense, bodyguard protection and other expenses.
When "national interest" is invoked, taxpayer funds can be used for military intervention or trade sanctions that enrich only a small sector of the private economy. Was the gulf war about Saddam Hussein's atrocities against his neighbouring countries, or was it about Mid-East oil? Are areas such as the Balkan states or Somalia of enough interest to justify expenditures for intervention?