Coinbase can do anything they want!
Also, I suggest it's people's own fault for storing their bitcoins at a place that does not give access to their own private keys. This fork has been a long time coming. Everyone and their mother was warned to keep your own bitcoins in your own privately controlled wallet (entirely offline if possible) to weather this fork.
Anyone who cries that Coinbase didn't do whatever is being childish and deserves it.