since this is a red board, im gonna say it is because they are sexually liberated
jokes aside (tough its still a valid point), i think it is because of the general attitude toward the very medium
in europe, comic books (BD) are an established object that no one see as strange and all strates of society can read
in the US, comic books are seen as something "inferior" that only children SHOULD and COULD read
effectively, adults that read comic books are ridiculed and any and all comics trying to do anything more... mature and intelligent is effectively discouraged from doing so.
all because theres this stupid ingrained idea that comics should solely cater to children and so anything trying to go beyond that is abnormal. some even fight against comic trying to be mature (think of the children!)
bottom line is, society in europe has matured
society in the US is immature