Update 10-7-13: Origin PC co-founder Richard Cary took to the Origin PC forums with a more detailed claim as to why the company decided to dump AMD graphics cards.
"The decision to take AMD GPU’s off the web was made based on support. More Specifically, support from AMD to Origin, and Origin to our customers.
Take their last chipset announcement for example. It was another hardware representative that informed us of the new Hawaii chipset. AMD never told us anything about it, nor shared a road map outlining its future integration. When we did get with our AMD representative, we were told that there was a new chipset launching, and we could watch the event live and get answers to our questions.
We also asked for samples to start our research and development process, something that is standard in the industry for bleeding edge integrators, and we were met with resistance there... When they did agree to send samples it was for [two] R9 270’s, rather than the R9 280's or 290's that we would be more likely to offer...
Moreover, this wasn’t the first time AMD had done something like this to us. Support from AMD to Origin hasn’t been where it should be or up to par like we get from other vendors. In my opinion it's due to our size. We don’t move enough AMD product for them to worry about us. You would think they would work with us to help grow the Origin-AMD business relationship."