Backflow prevention laws require the use of approved backflow prevention assemblies when there is a potential for contamination or pollution of the public water supply. To ensure backflow prevention assemblies are working correctly, they must be tested once a year. Uniform plumbing code 603.4.2 - Testing requires that,

"The premises owner or responsible party shall have the backflow prevention assembly tested by a certified tester at the time of install, repair or relocation and not less than on an annual schedule thereafter, OR more often when required by the authority having jurisdiction..."

Our water safety laws in the US stem from the EPA; specifically, they are from the Safe Drinking Water Act passed by Congress in 1974.

These laws provide guidance and regulation regarding the various ways drinking water can become contaminated. 

Laws pertaining to backflow address cross-connections, 

Montana's regulations require that cross connections be avoided whenever reasonably possible. If the elimination is not possible, the use of an approved backflow prevention assembly is required. 

 "A cross-connection on a public water supply system must be eliminated by the disconnection of the cross-connection whenever reasonably practicable. Whenever elimination of a cross-connection is not reasonably practicable and the cross-connection creates a health or non-health hazard, the hazard must be eliminated by the insertion into the piping of an approved backflow prevention assembly or device" (SOURCE)

Backflow prevention assemblies are important; they prevent all sorts of unsanitary and potentially life threatening pollutants and contaminants from coming out of our of our faucets. 

These assemblies contain springs, rubber gaskets, diaphragms and one-way valves - all of which can potentially break or become obstructed. That is why annual backflow testing is required. Montana law requires that, 

"A backflow prevention assembly or device must be installed and maintained, at a minimum, in accordance with the manufacturer's specifications." (SOURCE)

This is derived from the Manual of Cross Connection Control, USC. 

In addition, the uniform plumbing code 603.4.2 - Testing requires that,

"The premises owner or responsible party shall have the backflow prevention assembly tested by a certified tester at the time of install, repair or relocation and not less than on an annual schedule thereafter, OR more often when required by the authority having jurisdiction..."