The World of Warcraft licence agreement explicitly forbids the use of programs like Glider. Blizzard claims that Donnelly's enabling users to breach its licence.
However, Blizzard is also claiming that, because its licence prohibits the use of programs like Glider, Glider users are also committing copyright infringement every time they load their copy of the WoW client software into RAM to play it.
If the court accepts Blizzard's position, Glider users could find themselves liable for statutory damages which might start at $750 for every time they use Glider.
Blizzard's argument would also give software vendors the right to prohibit the use of any and all third-party software that interoperates with their products.
But WoW players each own their copy of the client software, and Section 117 of the Copyright Act says they all have the right to copy it -- that is, load it into RAM -- if that's necessary to use the software, which it is.
Blizzard argues that its WoW players don't own their copies of the WoW client software, but merely licence them. But courts have held that whether or not a user is an owner under Section 117 depends on the substance of a transaction, not just how one party describes it.
If you buy a copy of piece of software, install and use it on your own computer, and don't have to return it to the vendor when you're finished with it, that likely means you own it.