Possibilities....

I will try and avoid over-generalizations, but just remember that what I am about to say it probably cutting it close. Obviously, cultural norms are not universals and not everyone follows them, so please correct me if I am wrong.

Friendship.
In American culture friendship bleeds over and spills into others lives. If I am friends with you, then, a lot of times, I am friends with your friends. This is not because you already were friends with their friends, but simply because you became friends with them, you can be friends with their friends.
"Any friend of yours is a friend of mine."

In Japanese culture, friendship is based in groups and mini-clique communities. A single member freely moves from one group to another, but doesn't usually bring that groups friendship baggage with him/her.

Essentially, I see the fundamental difference as this:
Japanese: The group is itself a stationary unit.
American: The group is based on the person with which it is connected .

I could also be wrong about this, but that might be why I've had to get used to the hundreds of variations of the vague, "I have something else going on," or "I'm doing something tonight." Etc.. etc. Sometimes, to me as an american, that comes across as cold, but then I just tell myself, it's something cultural, and get on with it.

Oh, if it wasn't clear why I think its connected, let me explain. If my above theory is true, and Japanese groups are separate entities to which you cannot enter just because you are friends with one of the members, then telling you what they are doing is a moot point. However, to the American who is used to being invited to whatever, just because one or two of his friends are going, then not being told comes across as odd.

meh.. maybe I'm over thinking things again. I may be the one who is just too open and too willing to interact with anybody. I tend to agree with this:
"There are no strangers here, only friends you haven't met yet."