Unfortunately, this happens a lot. It's happened to me, as well, and as it was explained to me, the modem ownership status "switch" needs to be flipped in two places by two different departments, which often doesn't happen. Then, when an automated audit happens, the AI thinks the modem belongs to Comcast and generates the letter. Neither you nor I nor apparently the reps on the phone can fix it. A Comcast employee will need to address this; they answer problems in the order they are posted, so it may take a while.