I just sent for a replacement X (bad ear piece) and was not asked for a credit card for the replacement and I was told I would get a new one because I am in my 30 day window.
She did tell me she was not seeing any back orders but has heard of it happening and if that happens and it goes past the 30 days I can not return the device only get a certified like new after that.
Also have you ever been late on your bill? If you have even been a day late 1 time in I think it is 6 months then you can not charge it to your account and maybe that is why they are asking for a credit card. I had that happen once, I think I scheduled a payment not looking at the date and it was on a Sunday and it didnt go through till the next day and it considered me late, that has also happened to me with credit cards but always got them to take away the late charge (if you ask nicely they usually do)