This appears to be a mistake and probably will be corrected soon

Must CLAIM on a physical Xbox 360
Can WATCH on Xbox 360, Xbox One, Windows PC

On the 360 dashboard, navigate to the "Movies & TV" tab
Select the "Browse New TV" square
Select "Rick and Morty" on the far-left of the "New TV" listings
Once you're in the "Rick and Morty" section, navigate 1 tab to the right to "season 4"
Select the square that says "Get Season Pass / Free"
Make sure it is still free
If free it will say "Add to Collection" and not "Buy Season / Best Value", nor will you get a "confirm" screen showing payment/price information