Yes, It's true about the MK being blacked out for all of December. This has been a practice for several years that I know of.
No, I can't tell you why.
As far as MK goes, just take the tram and enjoy. After an entire day at the Magic Kingdom, the last thing I want to do is walk from the TTC proper to the Diamond lot. It's not that far, but still far enough when your dogs are tired.