It doesn't really help that Apple have a huge online digital music distribution business, and are trying to build an online digital book distribution business. Once you take this into account its hard to believe that they're really doing anything other than using their platform control in order to further their business interests elsewhere.

Gruber is entirely correct that Readability wished they had platform control, but I think he's missed the point that not every organisation abuses its platform control in order to attempt to remove competition. Its still right to call Apple out when they do this, especially when for 25 years their marketing has consistently tried to present them as a freedom loving, counter-cultural organisation.