I think chica is right. That rule you posted is in regards to road systems that are not freeways. This rule applies when say, you are going up a steep incline, there are 2 lanes for uphill direction, one lane down hill and you see the signs saying "Slower Traffic Keep Right".
My sister and brother in law have owned a driving school for 6 years now. I feel like I am in driver's ed everytime I drive with her. She drives by the laws exactly how they are written. The info about the center lane I stated earlier came from her. So I am only assuming it is true and I would bet it is coming from her.
The 3 (or more) lane system technically is laid out like this, (This is acording to my sis's teachings)
The right lane is for merging traffic coming onto the freeway and for exiting traffic. This is meant to provide less congestion and slow downs.
The center lane(s) is meant for all traffic at an even pace (aka the speed limit)
The left lane is for passing.
But even with these laws in place, one can't follow them acurately. Because think when the lane comes to a split and the signs tell you that the left 2 lanes will take you one way and the other 2 lanes will bring you the other way. In that case, which one is the legally proper lane to be in?
All in all, these rules will never fully be followed as the majority of drivers don't usually know these laws let alone know how to drive.
There are states where you can get ticketed for being in the left lane "Too Long". I believe WI is one of them. I could be wrong.