I don't think you're understanding the reality here.
http://www.automedia.com/Aftermarket...sm20021001aw/1
It's not the "written law" that a company can't void a warrenty for aftermarket parts use. It's general practice for some car producers like Mitsu and Subaru to be very stingy on cars like the EVO and the WRX STi which are limited in production over other cars. For the simple reason they are often bought for the purpose of modding and racing. Many of the owners will return a car to as close to stock as possible before bringing a car in for any warrenty for this reason. This is why some in the car world (EVOs, SRT4s, WRXs) had their liscense plates written down while at local tracks. It's excessive abuse and in this case if a dealer finds mods on a car that lead to extra HP or Torque then they generally will make a case to get out of the claim. It's then that the owner has to take the next step and fight it and priove his point. Most just drop it and pay. There is nothing illegal about that.