I have never heard about Marvel or Amazon specifically, but general info of this topic is pretty widely available.
What you are "buying" with on-demand services is the ability to stream a certain content as long as it is available to that service. If a studio or distributor such as Disney wants to revoke the streaming rights to a service such as Amazon they are within their legal rights to do so.
Were a content provider pull their content, the only legal recourse for the end consumer would be a class-action suit against the streaming service (Amazon) on false advertising grounds. They have convinced people they are "buying" content, when in fact, the consumer is "leasing" the streaming rights. Leasing is almost always a suckers bet.
If you buy something, you own it. How do you know you own something? You are able to sell it. The end-consumer can sell a blu-ray disc, but cannot sell a digital stream right (at least not directly).