This is not true. Civilian GPSs are only inaccurate due to atmospheric interference, and a nearby WAAS station can correct a lot of that out. Military GPS has the ability to average readings over the course of many minutes or even hours, so if you're setting up an observation station or an artillery position, you can get very accurate readings.

It's true that the GPS system DOES have the ABILITY to intentionally incur inaccuracies; the system is called "selective availability". The whole system is shifted around up to 100 meters, though they can make it be off by as much as they want. Along with the signal, they transmit encrypted corrections for the intentional inaccuracies, and military units are able to decrypt and apply the corrections.

Selective availability was turned off on May 1 2000 at the order of Bill Clinton. If SA were still on, a whole lot of civilian GPS use wouldn't really be practical.

The only remaining inaccuracies are variable signal delays caused by ionospheric effects.

One other thing that military GPS does that AFAIK no reasonably-priced consumer GPS unit does right now is carrier phase positioning. Time signal positioning gets you to within 10 feet or so (my handheld GPS often reads 11 or 12 feet error). Carrier phase positioning has the promise of getting as close as a few centimeters. However, it's significantly more difficult, and most people don't really care that much if they're 5 or 10 feet off.