1. In the US, yes. In other parts of the world, you'll see clocks that instead use a closer station (like DCF77 in Germany).

2. Indoor operation is part of it. Also, GPS receivers are more expensive and power hungry.

3. Depends on time of day, but it generally covers most of North America. There's coverage maps here: http://tf.nist.gov/stations/wwvbcoverage.htm