I can't take this map too seriously. California is Catholic only in that it has a lot of hispanics. This map paints with too broad a brush -- Some of the largest evangelical churches are in southern California, but you'd never know it by this map. It may have some usefullness, but it can also be quite misleading.
You are quite right. This map lists only mainline denominations and paints with way too big of a brush. For instance, the Catholic populations in Charlotte and Atlanta are growing rapidly and they don't even appear on the map. This is misleading and pretty much useless.