Did Google specifically work with them on it? What are they getting from supporting it now? Are they just being paid?
How will advertising work on Accelerated Mobile Pages? (...) As part of that, those involved with the project are also engaged in crafting Sustainable Ad Practices to insure that ads in AMP files are fast, safe, compelling and effective for users.
If Google should at one point choose to block all non-AMP-compliant ads in Chrome, it might be a good idea to get into the boat early when "AMP-compliant" is still in the process of being defined.
I expect that they've worked with people to figure out what makes the most sense to actually build and so the BBC and Buzzfeed would have had a bit of time before the announcement to start making the pages.