Become a MacRumors Supporter for $50/year with no ads, ability to filter front page stories, and private forums.
It's just like how they don't allow At&t or Verizon to put logos on the iPhone. They invest a lot in design and they don't want anything to distract from it.
 
hardware manufacturers put stickers as part of their partnership agreement with Intel/AMD/Nvidia etc. originally Intel used it as a form of advertising while the hardware company got a percentage kick back. eventually other semiconductor companies followed Intel and that practice just stuck around.

Apple doesn't have stickers probably because Jobs valued design. regardless of reason, they created a different partnership agreement with Intel, AMD and Nvidia to avoid the stickers.
 
Register on MacRumors! This sidebar will go away, and you'll see fewer ads.