2006 Other Suzuki Models Faulty wheels

All 4 tires of my Swift have worn out on the outside only (all need to be replaced). It's a front wheel drive but the tyres at the back are the worst. I took it to a Suzuki dealership who is basically telling me the wear on the front is normal and I should only get 20000 miles out of them. They want me to replace 2 tyres at the back and arrange through them, at my expense, to get a cast and camber on the rear wheels. There's clearly a problem with the car but they're shirking all responsibility, can you advise?