Mazda 3 2008: Clogged DPF

The DPF on my 2008 Mazda 3 diesel has failed through being clogged. The car is under warranty but Mazda has refused my claim because the DPF cycle was not reset at each oil change, some of which I did myself. I had not been advised when we purchased the car, or at any subsequent services at the dealer, or indeed when we purchased oil and filters at the dealership that Mazda has a requirement that at each oil change the computer (DPF) must be reset by the dealer. I don't agree with Mazda's rejection of my claim, which I believe is valid. The car has been serviced as per maintenance schedule, the major services have all been carried out by the dealer, oils and filters for changes other than major services were all sourced from the dealer, and we have extended warranty time on this vehicle. How can a company require that you return it every 10,000 km to have the DPF reset?