This wasn't "theft" (aka misappropriation) of trade secrets. The guy did nothing wrong. He didn't publish the materials and merely came across them incidentally. He had no obligation to return them.
Would you be willing to take that to court against Blizzard's army of lawyers, all of whom are no doubt far more well-versed on copyright and trade secret law than any random lawyer you could afford to defend you?
Supporting the release of stuff like this isn't all that far off from supporting piracy - it's free distribution of something that isn't yours without the permission of the copyright holder. Yeah, the game is 20 years old, but that doesn't make that source code any less protected than it was when it was first written.