well then did you have them rotated at the proper intervals? did you keep the pressures correct? is your alignment within spec?
this is quoted directly from cooper tire us
No manufacturer or dealer can guarantee you a certain number of miles from a given tire. Driving habits, driving conditions, tire and vehicle maintenance all play a part in the tread life of a tire and all differ with each purchaser.
if your tires only lasted 30k then you should have them prorated. but you have to have proof that you had them rotated, kept at proper tire pressures, and proper alignment.