They do so all the time. That's what it means to live in a society.

In some cases, yes. This is a key part of a society. Your desires are not absolute and should not be obeyed to a T just because they are yours.

In any case your desire to not donate your organs to help others can and will be respected.

Why is it only Americans who have these strange 'screw everyone else' tendencies and opinions which seem to fall apart at the slightest scrutiny?

Why not? You can't punch me. You can't take my stuff. You can't build a roof over my property.