Now, this is definitely not something that can be said about all Americans. However, there is a large portion of Americans that have some very strong religious beliefs. And because they grew up in an area where just about everyone else they knew also had those religious beliefs, they might not be very aware of other religions. This can be potentially problematic when they are traveling to countries where the majority religion is not theirs.
If you want to avoid overly touristic behavior while traveling (and just have common decency), be sure to accept and understand other belief systems while traveling.