I think that it's shady that they kept his BTG because we don't know if those coins were or weren't sold by coinbase. It would be free money for the company if they were to take those coins at any point and sell them without telling their clients. We're talking about millions of USD here not just this guy's coins.
At the same time if they were forced to fork BTG by court they'd have to install the software for every single BItcoin fork possibly compromising their security. People would start to demand Coinbase pays them their Bitcoin God or whatever...
I don't see Coinbase or any centralized exchange liable to pay all forked coins, no matter what they do with forked coins. Forks like Bitcoin Gold (BTG) have zero impact on Bitcoin and its market. Go and conduct quick survey on Twitter and ask 'which coin do people consider real bitcoin'? Bet me, BTG won't get even 1 vote in hundred. Also, to pay the forked coins to the users, Exchange have to integrate forked coin on their platform which indirectly imply that Exchange is supporting that coin.
Also, not to forget, Coinbase has clearly stated in ToS:
5.11 Advanced Protocols. Unless specifically announced on the Site or via an official public statement of Coinbase, we do not support metacoins, coloured coins, side chains, or other derivative, enhanced, or forked protocols, tokens, or coins which supplement or interact with a Digital Currency we support (collectively, "Advanced Protocols"). You should not use your Coinbase Account to attempt to receive, request, send, store, or engage in any other type of transaction involving an Advanced Protocol. Our platform is not configured to detect and/or secure Advanced Protocol transactions.