Dtac wants to see my Bankbook!?

I have just been to Dtac in Udon Thani for some assistance with my phone. They helped me but they also told me that they wished to update their information about me. They liked to see my Passport (OK) and they they wanted to see a bankbook of mine??!!
Why on earth do they ask to see my bankbook?? I have been their customer for more than 15 years with a postpaid account and never missed a single payment.