They're not sued because many of them are based out of third-world countries. They're selling you the exact questions and answers on the exam, which is no different than buying a professor's final exam stolen from his or her desk.
Those that were based in the US have been sued and shut down. The largest one overseas has been sued by Microsoft, but nothing will probably ever come of it because they're in a third-world country.
I know you probably didn't know you were using material that is considered cheating by Novell, Microsoft, Cisco, etc. However, I thought you would want to know for next time, because you can have your certifications revoked if they find you've used them. If you want to take the moral high ground (and be a better trained tech in the process), stick to legitimate practice exam providers in the future. These include:
Transcender
Self Test
Measure Up
Boson
I've had 5 years of experience writing for one or more of those companies I listed. In fact, I'm creating my own practice exam company to compete with them. However, I must respect AT's advertising rules and not post the name of our company. We hope to have our first products out by next March or April.
Best of luck to you!