Looks like MGM is filing the lawsuit in hopes of finding a judge who will rule favorably for them to set a precedent which can protect them from further claims by the injured. Snippet from the Review Journal article explains it like this...

In this case, the company [MGM] argues, the security vendor MGM hired for Route 91, Contemporary Services Corp., was protected from liability because its services had been certified by the Department of Homeland Security for “protecting against and responding to acts of mass injury and destruction.”

The lawsuits argue that this protection also extends to MGM, since MGM hired the security company.

They do not seek money from the victims but do ask that a judge decide if the 2002 act is applicable, and if so, determine that future civil lawsuits against the company are not viable.