Why do comics people and publishing folks in general worry about more and more power accruing to Amazon? In a nutshell, it's because there is expected to be increasing pressure on the retailing giant to show a profit, and one way in which they may attempt to become more profitable -- according to past example and overhwelming conventional wisdom -- is applying pressure to their partners. This includes but is not limited to the publishers for whom they are the most important or near most-important retail outlet.
Here's a succinct, plain-spoken summary of their latest conflict, which might read as loaded but I don't know that I've found anything that directly disputes what's going on here. That in itself is kind of scary. These issues are something to keep in mind with this company in particular and large companies more generally: profit has to come from somewhere, and that might disrupt or at least put strain on exisiting systems.