Lexus IS350 premature brake disc wear

The brakes on my 2011 Lexus IS350 F Sport squeal very badly under light application, e.g. when lightly braking when coasting to a stop. I find it embarrassing, so I brought it up with my dealer, and they machined the discs. The noise came back. This time they said that I would need to replace rotors as part of wear and tear at my cost, I'm looking at $800-plus. This was also suggested initially when the problem first arose, but I got away with machining the discs. Apparently there is a section of the disc/rotor nearest the centre and outer edge that is higher than the rest of the surface and the brakes make the noise when contact is made. Should this not be a warranty issue? I expect it at 60 or 80,000 km, but to start from 20,000 km seems faulty from the manufacturer in my eyes.