There have been some recent threads about this. I've also seen it in sushi restaurants.
Clearly, a lot of Americans put a lot of wasabi on their sushi. Some mix it with the soy sauce and dunk the rice side; some smear it on top of the fish, whatever.
From posts by Japanese people and others who have been to Japan, and also from observation, most Japanese put a LOT less wasabi.
Most of the fish and other sushi items have delicate flavors which are overwhelmed by wasabi. Very little Japanese food is spicy (I haven't actually had ANY Japanese food that was spicy, but there may be some....)
But perhaps Americans with a little bit of knowledge of 'foreign' especially Asian cuisine think 'the foreigners always put more hot stuff than we do' and
gneralize to Japanese?