Alas, the software is tied to Google's platform. Just releasing as open source won't do. (That's why they didn't open source Google Reader. The source would have been both useless for running a Reader clone, and given away lots of trade secrets about Google infrastructure.)
I don't doubt there is an a-symmetry between the number of machines / GPUs they throw at the problem during a match and during the run-up to a match but even so they will have to have some margin of error if they expect to win in the first place and besides that whatever that pile of hardware is it, the infrastructure required to run it and the people involved are not free.
If they would release the (trained) AI to everybody, that would prove that the training phase is general enough to beat any player, not just one.
Google doesn't have to pay for the CPU time. Ke Jie can find sponsors if he pretends he can beat AlphaGo.
(Edit: clarify)