I'm glad that you react to those reports, but I do really hate that your usual policy of advising to switch chat settings if you don't like messages you receive is going to be a default setting forced for everyone...
I would assume that they probably cannot detect who have knowingly set (or left) said setting to "anyone" and who haven't understood that the setting exists or don't know what it does.
In this case, I would take GOG's side and say that it's better to change it for everyone with the setting to that value than to not change for anyone (those who know about it can change the setting back, and those who don't are safe from spam). It's a sensible default, in my opinion, and it's not that difficult to change, should you want to. (I'm still opposed to having "install Galaxy?" defaulting to "yes" in the stand-alone installer, mind you, because advertising for the client is already all over the damned site, so it's practically impossible to miss.)
On a (un)related note: Is the GOG forum even worth spreading spam on? Seriously?
Apparently, else they wouldn't continue.
Do realise, though, that only very few messages need to actually get to gullible enough targets. The messages barely cost anything to send, so one hit in a million is probably a gain.