Rear tires gone at 11k

370z TT

I'm at work but once I get the chance I'll post pictures. I honestly couldn't believe when the dealership told me that 3rd gear bucking was due to bald tires. He told me it was due to burnouts and I told him I refrained from doing burnout or doughnuts because I was saving up for a new set of rims and tires. I even called my mechanic and he even told me that the worst tires could withstand at least 20k miles.