Anyone can create an altcoin fork of Bitcoin at any time, and they can do it as many times as they want. Coinbase may not even know about it.
ABC isn't the first fork to award coins to existing Bitcoin users, there have been several others like Clams and bitcore (https://bitcointalk.org/index.php?topic=1883902.0) (how deceptive these jerks are...).
In spite of the seriously short notice coinbase had done an admirable job taking a clear and consistent position and communicating it to the public. It's a position that can be applied to any further such events. In ABC's case they took serious bludgeoning just to get the most basic replay protection into place-- and they still have many unnecessary conflicts with Bitcoin: duplicate address types, duplicate p2p port, duplicate p2p magic, duplicate data dir... all making it unnecessarily complicated and risky for people who want to use their altcoin.
Consider if they went another way-- what about the next one? and the one after it? People could effectively flood them with them, a fork a week-- forks that contain malware backdoors that steal Bitcoin keys, forks that will never have more than a few dozen users. forks that are only publicly announced months after they split off (but pretend to have been public all along).. etc.
No one can reasonably keep up with that. If you want to chase some of these silly things, that is your decision-- one which you have the freedom to make by keeping your Bitcoin's in your custody. Expecting other people to deal with those potentially unbounded costs on your behalf wouldn't be reasonable.