This Reddit comment explains the concept of Minimum Advertised Price well
Basically there is an agreement in place between MS and Amazon in place to not "advertise" the product below a certain price (in this case showing the price before the customer's made the minimal commitment of interest represented by adding to cart) because they want to
A) Protect the perceived value of the product
B) Protect their own retail outlets which probably won't sell below that minimum advertised price anytime soon
It's a very "light" form of price fixing