It isnt law anywhere. That is my point. If you're traveling in the rightmost land on a three lane highway you are making it difficult for others to get on and off. The middle lane is where you should be.
I drive the speed limit. If I did that in the middle lane, people would be passing me on both sides. The right lane is for slower traffic. Lanes to the left are for passing. Merging is the responsibility of the drivers merging.