Why Port Side of Channel?

[Lodesman]

Rule 9 - Narrow Channels Return to the top of the page
(a) (i) A vessel proceeding along the course of a narrow channel or fairway shall keep as near to the outer limit of the channel or fairway which lies on her starboard side as is safe and practicable.

That's the rule straight out of US Inland rules - so why do so many boaters insist on running down the wrong side of the channel (their port side)? Is it just American boaters, or are these people all over the world? WTF?