I'm pretty sure that's not the way it was presented at all. Modifying original works is actually an allowance of copyright, not a restriction. It's called Fair Use, which states that "transformative" uses of copyrighted material, or "those that add something new, with a further purpose or different character, and do not substitute for the original use of the work," are "more likely to be considered fair." [1]
Extracting the user's data out of the page is what's _allowed_; since that is the part of the page that Facebook does not actually hold a copyright claim to, I'm pretty sure you could make the claim that extracting the user's own data out of the page into a structured format for the user's own use is transformative.
The part that wasn't allowed was literally making an ephemeral in-memory copy of the page as-is, which included Facebook's copyrighted HTML.
(I am also not a lawyer)
[1] https://www.copyright.gov/fair-use/more-info.html