The bitcoin protocol doesn't (on purpose) have a way to reverse transactions. Unless coinbase isn't actually moving money, it won't be able to reverse anything. Since they have their own database, though, they could provide chargebacks for any transactions they haven't committed to the blockchain, though I don't know if I'd want to use that as a merchant.

Right; bitcoin does not, but Coinbase is just flipping bits in their database, since these transactions are off-chain. So, just a question of whether or not they support chargebacks, and to what extent. The issue will come up sooner or later, if Coinbase sees more mainstream usage.

In which case people who has compromised accounts wouldn't send it within coinbase's system, they'd sent it on the real network where it can't be reversed.