Disney is selling the Magic Keys based on customer trust. People who bought the keys trusted Disney to allow them to get in whenever they are not blocked out, as long as they make reservations. In reality, the Magic Key means Disney can theoretically block out every single day of the year from all Key purchasers -- they are completely at Disney's mercy.
Now that Disney is blocking out dates, it's a breach of implicit contract. Essentially, the keyholders got less than what they thought they paid for, and there's nothing they can do about it besides refusing to buy more keys in the future. Disney is betting on people getting used to the new system and not wanting to live life without Disneyland. That way, even though they might complain now, they will continue to purchase in the future, and current complaints will eventually fade out and be forgotten.
The current situation is only the beginning, being that Disneyland just started selling Magic Keys. As time goes on, more people will buy, and reservations shortage will only get worse. Because Disney will certain never stop selling Magic Keys. If the customer doesn't read the fine print, it's their problem. The reservation difficulties about Magic Keys will come out so that the fine print will eventually become common knowledge. By then, people who buy Keys will no longer complain, knowing what they are getting into.