1. It hinders competition. You can't really provide a marketplace thay competes with Amazon by having lower prices (for example by taking a smaller cut from sellers), because sellers aren't allowed to list lower prices on your marketplace. Or alternatively, you can't compete by selling directly from you website, because you can't offer a lower price than on Amazon.
2. It means even if you buy directly from the seller, you have to pay a higher price because it is listed on Amazon. In effect people who bu directly are subsidizing a lower price for people who buy on Amazon. This applies to the credit card case as well.