Besides being possibly against their own interest, as less profit for the authors make them less likely to create new books/products, and therefore provide less things for sellers to sell.
But theoretically they could always sell some e-books/products hidden from the author and increase their profits in the long run, with the vast amount of content from many different authors.
Again, the best counter argument I can imagine is that it would be safer and simpler to just increase their rates "accordingly" and spread this "extra" profit all over, to avoid the risk of a programmer pulling off an "Assange" whistle-blowing move and damaging their credibility, possibly leading to legal action against them.
The more concrete way to be next to sure that nothing like that is happening would be if there were some sort of trustworthy third-party auditing of their mechanisms. Is there such a thing?