1) very small % of WoW players heard about glider and even smaller % ever used it
2) huge part of WoW experience is well balanced economy, bots (and gold sellers) really screwing economy - hence negatively affecting every WoW player.
3) from my understanding how glider works - it is VERY hard for Blizzard (or even impossible) to automate process of detecting accounts which uses glider.
Bottom Line: Blizzard has only 3 options:
1) Ignore the problem and have WoW experienced worsened for many "legit" users,
2) Try to make it impossible to implement glider like functionality,
3) Sue any company which trying to bring gliders to the WoW.
Blizzard can not live with option #1 - unhappy customers liable to leave. They trying to work on #2, but when you run on something as insecure as Windows, it is quite futile. So the only thing left - sue them.
And they do have good case against MDY. Glider work in very much the same way as some more sofisticated viruses (the only difference, is that Glider does all "bad" things in memory only).
A company is known by the men it keeps.