Copyright concerns copies. When Comcast injects their code in a page before delivering it to me, they have distributed a copy which is subject to copyright laws.
When I download a page and modify it for myself without distributing the modified copy, it is not subject to copyright laws.
I'm not sure they are equivalent. One is being done by someone else perhaps without your knowledge (most people are not tech savvy and won't even understand what Comcast has done). The other is being done by you explicitly to your own "copy" of the work. I don't think the ad-blocker argument would hold up in court.
But this line of reasoning got me thinking...I've seen some pretty loose interpretations of the CFAA over the years. I'm not sure what the law says about Comcast's privileged position as an ISP, but I would think that in most cases, specifically altering data between two networks counts as unauthorized access, no?
In this case, one could argue that adblocking is like using a marker to blackout sentences you don't want to read in your personal copy of a book or newspaper.
The same as using a blue light filter on your computer (modifying the output of every program, copyrighted work, website, and text) vs wearing blue-blocker (yellow) sunglasses to the movie theater or library.
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.
This can be a crummy, anti-consumer practice without having to invoke copyright.