How many miles were on it when you bought it? Perhaps the dealers used it to screw around with and test drive it and they launched the crap out of it?
I would still fight it however. Even though its a wear part, mitsu has to prove YOU are the one that did it after it was purchased in order to deny the claim.

 

Not trying to bash or anything but many many people on here beat the crap out of their cars with 300-400hp on a stock clutch for many thousands of miles (including me) and have no issues. Either "A" you were launching it hard a lot, which will definitely wear that clutch down, or "B" you dont know how to use the Evo clutch properly. Either way, I don't see anything wrong with Mitsu turning your warranty down, and besides, if your that hard on your car stock stuff is just gonna do the same thing again shortly. Do it yourself and do it right.

 

This clutch looks fine. I see no abnormal wear at all, granted I cannot measure thickness.

 

or "C" there was a defect that caused the clutch to wear improperly.

If he's that hard on the car, then he knows it and wouldn't be here posting. Doing that much damage that fast isn't something that happens without you knowing. If thats the case, why the heck would he be trying to get another stock clutch, knowing he'll just blow it out again in a few short months? Wouldn't he be going for an Exedy or something?