Because Penguin Random House is one of the "Big Five" publishers. I can bet the house that Amazon treats it as a key strategic account within their Books department. They have to deal with them face-to-face regularly. It would be grossly negligent to do this without their consent, not to mention illegal.

It's as if Amazon started selling pirated Nintendo games. Someone from Nintendo would be on their arses in very little time. And while I know very little about the publishing industry, I worked as a PM for Nintendo for four years, and can absolutely guarantee that it would go down like that.