My Platina had some manufacturing defects. Review on Bajaj Platina 100

I had purchased my bajaj platina in 2006 and had done the periodical service without fail from bajaj authorized service center till 2010. By the beginning of 2011 my bike engine started to create unwanted sounds and it become so unbearable that I needed to take to the service center. After the service I had driven it for two days and the sound produced by the bike is like some hammering sound on a metal sheet. I took it back to the service person and they charged me Rs 11, 000.00 for the engine work. I asked them why such an issue had happened as I am spending exorbitant money for the last four years to service my bike at the authorized center(almost Rs 12, 000 in total for a distance of 30, 000 kilometer). They just told me that the reason is some manufacturing defects.

My question is that if there is some manufacturing defects then why cant they rectify it themselves. Why shoul they pass on the burden to the customers. However now the bile is in running condition, but not as smooth as earlier. Giving a mileage of 60 km. In the last 6 year I almost spent the cost of the bike itself to maintain the bike in a good condition within a total travel done below 40, 000 K.M. just pausing it as now for buying a new bike minimum I have to pay 70K+.