Honestly, I think it's in their right to do whatever they want with it. It's nice that they kept it open source entirely until now. Even SQLite, which is the open source project par excellence has additional modules that are not publicly available (encryption, compression)
"...if someone pays your fee and gets a copy [of the source], the GPL gives them the freedom to release it to the public" -- from GPL FAQ. Which means, any paying customer has the freedom to request the sources and then publish them. Did MySQL folks miss something?
I don't see a problem with it. They are still providing the source. What is the issue? IMO, it isn't any less in the 'spirit of open source' to do something like this.