Tire Tread/belt problems of the 2000 Suzuki Grand Vitara

The vehicle was purchased from farrish automotive. However, customer service in service dept. Was so poor that I started taking it to woodbridge-Suzuki. Not long after purchasing vehicle, I noticed that tires did not have much tread. I've had all required services since owning vehicle: tires rotated, oil change, recommended maintenance at dealer. During earlier visits, I expressed concerns to dealer because the tires didn't have much tread, and they said the tires were fine. The dealer which I purchased vehicle from said the vehicle was a demo, it had 7,000 mi. On it when I purchased. I've only put 16,000 miles on it. The dealer said the tires are not covered under warranty. However, service manager at dealership said he would take it to tire company to see if they would prorate. He took vehicle to ntb, I don't understand why he didn't take it to firestone. He claims firestone has nothing to do with bridgestone. I'm basically getting the run around. I need direction on what steps to take next. I thought that ntsa would be a good starting point. It's not right that I have to pay out of my pocket for a problem that's not my fault. Thanks for your time in advance.