Anti-back-flow, anti-siphon, valves are required by plumbing code in many areas.

The local authority would like to have them on all residential lines but so far, largely due to cost and the objections of the home-builder's lobby, they have been limited to commercial properties. Near about every strip mall you drive by you can spot an above-ground turn-off and anti-siphon valve assembly tucked in the shrubs somewhere.

Of course such regulations are bait for the 'government can do no good' crowd and is often characterized as 'the jack boot of tyrannical government on the neck of free enterprise and the common man' but I really don't think they have a case. Yes, someone will sell you the valve and installation and, like true capitalist, make a profit doing so. Then again, if you consider the cost of contaminated water on just one person you can pay for a whole lot of valves.

Contaminated water isn't really much of an issue for most adults but it can be fatal to the very young, the very old, and immune compromised persons. It can, and does, kill infants and old people. Even if they live without any permanent disability the hospital costs can be in the hundreds of thousands.

The price of freedom is responsibility. If you wish to enjoy the benefits of collective, municipal, water supply (tasty, cheap, safe water) you have to comply with rules designed to keep the people using the same supply safe.