I think the logic here is that the code to deliver an ad is protected by copyright and your modification of that code is considered a derivative work that is protected under copyright law.
But that's not what happens at the browser level.
The HTML code is sent, whole cloth, to the browser. The browser inspects the code, you know, to do browser stuff.
During this inspection, the code is put against the ad block rules. Nothing is modified. If the code violates some sort of logic, it doesn't get rendered properly.
Hell, the opposite argument is probably more damning. Say you have this literal HTML:
<html>
<title>I use arch</title>
<p>
Btw
hello
World</html>
You could argue the browser is NOT showing your code the way you intended (e.g. "Btw hello World" being rendered though I'm not sure if spaces would be there or not).
At the end of the day, unless you send your webpage as an image, you can't guarantee how the browser will render it.