The concept in economics is that of a "free rider" - someone who partakes of a public good without paying for it because it is available to all. The classic case is a lighthouse. To be a bit more sophisticated, all taxpayers do pay for security but most do not feel (subjectively) the cost so, in terms of military security, I would argue are free riders.

While the US has, historically, had a volunteer regular force, we usually had an involuntary militia force. Under the Militia Act of 1789 ( exact date?) all male citizens age 16 to 45 (or 60?) were members of the militia with drilling obligations and call-up in times of emergency both state and federal. The creation of the National Guard about 1916 changed the involuntary nature of the militia to a voluteer force.