It's Law.

Hi JonWise. This is directed to you, or anyone else that thinks that the yellow lines in the middle of the road are just a simple "guideline".

"In general, the law requires that we drive on the right side of the road. When passing is allowed, we usually pass other vehicles on the left. Passing on the right is permitted only in limited circumstances, and should be done only when necessary and safe to do so.

When passing other vehicles or changing lanes to avoid hazards, do so with caution and only when necessary. You must not exceed the speed limit to pass another vehicle...

PASSING ON THE LEFT
The left lane is usually used for passing other vehicles. However, you may not pass a vehicle on the left if:

• Your lane has a solid yellow center line.
  
• You cannot safely return to the right lane before reaching a solid yellow center line for the right lane.
  
• You cannot safely return to the right lane before any approaching vehicle comes within 200 feet (60 m) of you.
  
• You are approaching a curve or the crest of a hill on a two-way road and cannot see around or over it.
  
• You are within 100 feet (30 m) of a railroad crossing on a two-way roadway.
  
• You are within 100 feet (30 m) of a bridge, tunnel or viaduct on a two-way road and your view is obstructed."
  

I got all this, and more, from Here.
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