Not really the same thing at all. Comcast is modifying content, and profiting off a service that delivers that derivative content. That is clear willful infringement. User modifying their own content locally is not infringement, since the content is not redelivered or sold.

You're free to tear up your copy of Harry Potter once you buy it from the bookstore, but you're not free to (as the bookstore) add a prologue to every book and sell it as Harry Potter by Comcast.