Unless you are claiming that Comcast injects their own ads into your traffic, what are you even saying? You request a page from a server, they deliver that page to you, in whole as requested- The fact that there might be advertising on that page is due to the server/site owner. Why should Comcast have to not charge for content that you technically requested (although likely didn't want).

If they are injecting ads (which I have seen through other providers, but I believe it wasn't related to Comcast) then your point is valid. At that point you are being delivered packets (large video-laden amounts) were not requested, and being charged for them. As a side note, I believe Verizon was doing this to mobile customers, not sure how it ended though.