delayed without compensation

On my flight back to Capetown, the flight was delayed 3:42 h in Istanbul. There is a compensation of € 600.- for a flight like this. Turkish airlines, so far, confirmed the delay but refused to pay the compensation. Their explanation is that i boarded the flight ( with the delay ) and therefore is no compensation. That´s ridiculous. Why are there rules for delayed flights and they ignore it ?. The delay is not an exception. I´ve got 4 other friends they experienced the same and also battle now for the compensation. Never again with Turkish Airlines, even the flight was good.