Hmmm, well there are a lot of potential reasons ranging from intimidation, to the hackers backing down, to a lack of evidence as people have pointed out.
Strictly speaking given the lack of information about the situation I'd tend to believe that whatever the reason is, is not beneficial to Blizzard. If they cut a deal or hired them or something you'd probably see some evidence of it. On the other hand if they accused the wrong people or were unable to prove anything, they probably wouldn't admit to that. Ditto if the group had enough anonymous members of sufficient skill to tell Blizzard that if they continued they'd totally wreck Blizzard's online infrastructure and cost them millions upon millions of dollars for the sake of sending one guy to jail (allegedly such things have happened).
Oh yes, and of course the most likely alternative that there is some kind of legal loophole in play used by a defense lawyer. Strictly speaking there is a lot of pro-user legislation in place. If they played the defense right, things like the EULA are actually relatively powerless since they fall under various "fine print' laws (or they did years ago), where basically anything that is to be legally binding has to be relatively short, consise, and easily understood by the everyman. Things like referring to other documents, using asterixs, and similar things can actually reduce the strength of a document if your lawyer knows what they are doing. Basically TL
R can be used (or could have been used) as a legal defense. What's more things like EULAs also fail to have a lot of power because they are only presented AFTER you make a purchuse, as opposed to being printed clearly and consisely on
the package.
The bottom line is, we'll probably never know. It's probably something fairly straightforward though.