It's an opportunity cost: if I missed out on some big trades while the price was high due to their decision to withhold the keys, I'm rightfully annoyed by it.
Of course leaving legality out of it, it'd also just be a smart thing to do because it makes their customers happy and maintains goodwill. Coinbase is fairly respected in the crypto space and making their customers happy during a turbulent period can only be good for them in the long run.
They will capitulate eventually. Same thing happened with ETH/ETC.