They clearly stated, well in advance, through multiple channels, their intention not to support BCC. They specifically told people to take their BTC off CoinBase if they wanted BCC.
Doesn't matter. The fork occurred, so they can either give the duplicated amounts to their customers or keep them to themselves and who knows, sell all of the BCC somewhere else for all we know. To me, the latter just seems to be unrightful profit for coinbase.
As a customer, the first notification of this was sent to me was around ten day before the fork, I don't think that would be considered as 'well in advance'.
I don't think they could have been any clearer.