I believe it comes from Executive Order #12919 which is an amalgum of a bunch of other presidential EOs going quite a ways back.

http://www.uhuh.com/laws/eo12919.html

This is the same controversial EO that gives FEMA nearly absolute control over the United States if another 9/11 happens.

This EO allows the government to confiscate what ever it wants (not really but close) in the interest of appropriately allocating national resources in the case of an emergency. That includes water, food, your car, and YOU!