Ad blockers do literally the reverse, they don't inject anything, they sit on the outside and prevent unwanted resources from loading.

Also it's fully legal for the end user to modify stuff on their own end. And the information in the filter about the website structure is functional, not expressive - no copyright protection of function.

To claim copyright infringement for not rendering a website as intended due to filters also means it would be infringement to not render the website correctly for any other reason - such as opening the website with an unsupported browser, or on hardware with limited support, or with a browser with limited capabilities - or why not because you're using accessibility software!