Cooper tyres sidewalls problems

I bought 8 cooper at3 tyres about a year ago i have done 10000kilometers on them and om 7 of them the sidewalls are cracked now they tell me i must pay in 50percent because i have used 50percent thread how can you use 50percent thread in 10000kilometer i have goodyear wranglers on my other bakkies done 80000 kilos and tyres is not even 50percent and no signs of cracks like the **** cooper tyres i will advize anyone not to buy cooper tyres as it is not suitable for south Africa roads with no customer care.cooper sa said they cant help me because i no longer have the invoice so i guess the cracks in the sidewalls is the invoices fault.i will rather spend the r20000 for new tyres on making stickers and banners to advertise how bad the coopers at3 is