Pretty much all of California is stuck at home, and California has 40 million people... that would be the 8th largest country in Europe.
Also, California is mostly fairly urbanised and well-off. If Germany was the only place in Europe where people were being discouraged from leaving the home, this would be less of a concern. This is really targeting places, especially rural places, with poor infrastructure.
Also, there are very large parts of California that are rural.
That's about 5%, which is pretty low.