Goko can thwart the extension any time they want to. It's running in their JavaScript namespace and they can even download the source code. We couldn't "win" an arms race with Goko while keeping the extension public, nor would I personally be interested in that fight. They can also deny you service for using the extension or for pretty much any other reason, according to their TOS. I don't think they're likely to get too aggressive on that front, though. Even ooksoo still has his account.
Other than the tag line (and yeah, that was probably a bit over the top), I can't really think of any reason for Goko to resent the extension. It doesn't provide an unfair advantage, nor does it draw additional server resources. Most of its features have been suggested on getsatisfaction and gotten responses like "yeah, that sounds nice and we might do it someday." If Goko started worrying about game mods, they'd do better to target unofficial ones, like a point counter that doesn't offer #vpoff or an exploit of the kingdom-fixing vulnerability michaeljb reported.