There's a difference between culture and religion. Having a 10th-century depiction of the crucifixion in my passport doesn't imply that I'm religious, but it does remind me that I come from a country with a long and rich history and culture. In Northern Europe religious artifacts are primarily seen as cultural heritage, not as a way of showing your devotion to a god. I can't speak for Southern Europe, but from my limited experience of living there it doesn't seem very different.
"American church attendance and religious belief is not off the European scale if one compares it with Europe's Catholic regions. A smaller percentage of Americans consider themselves religious than the Portuguese and Italians. Proportionately fewer Americans say they believe in God than the Irish and Portuguese."
The article compares all of America to a small very religious part of Europe. That would be like saying that Americans produce 20 times as many cars as Europeans because the average number of cars produced per capita in Detroit is far above the average number of cars produced per capita in Europe.
The US is the country with the smallest percentage of non-believers (3%), and as stated in the article Portugal and Italy aren't far behind(4% and 6%). The article doesn't however mention Sweden or France that are 46% and 43% non-believers.
Source: http://www.swivel.com/graphs/show/8244121
Spiegel should really know better than manipulating numbers like this.