I've concluded there were a few benefits HBO got from their close relationship with cable companies: 1) The huge customer base that cable companies have 2) The advertising that cable companies provide 3) The distribution of content that cable companies provide
I always felt the release of content to Amazon Prime was an experiment and judging by this news, I think that experiment was deemed a success. I suspect that HBO concluded that Amazon is more than capable of distributing the content and that there are plenty of customers willing to pay for it when it's delivered by a means other than the cable company.
I still suspect that Amazon will play a role in hosting and distributing content for whatever "HBO Go" becomes one year from now.