(These photos come from this gallery‘ of backstage Disneyland photos.)
Why the mysterious basketball court?
Inside of the Matterhorn structure, the attraction only takes up the bottom two thirds of the mountain. The top one third was empty. There has always been a tradition to have Disneyland Cast Members climbing to the top of Matterhorn to wave to guests and occasionally play music.
These mountain climbers are the only people to ever be in the top of the mountain (it is their break room), so Walt Disney asked them what they would like to have put up there. Long story short, they requested a basketball hoop.