TL;DR; It was considered to be legal because HiQ were scraping public profiles.
This case may be a little bit different because these scrapers seem to target both public and private data.
Also see: https://medium.com/@tjwaterman99/web-scraping-is-now-legal-6... (which mentions a facebook example very similar to this)
Those individual users likely go against FBToS in doing so - but the browser extension maker is not a party to those ToS.
According to the article, it's the reverse, the individual users are acting as paid agents of the scrapers, a proxy giving scrapers access to data through the users' access to Facebook.
I wonder what legal principle gives Facebook standing to sue the makers of the extension.
Each user only chooses to install the extension, the extension makers choose what data to scrape and I'm sure at least some of what's scraped truly belongs to Facebook, not their users or customers.
If a Facebook account can be considered a technological barrier protecting intellectual property, they could sue under DMCA anti-circumvention provisions. The users installing the extension could be accomplices but they're not really the ones scraping and collecting the data.
It would also likely make screen readers, screenshots, and many other things illegal. I don't think it can be considered a "technological barrier", but who knows what courts will decide.
That said, I'd be surprised if that's the angle Facebook is taking - because they did (and still do) the same thing with their phone apps collecting ("scraping") address book records and uploading them to facebook servers -- and that would be an estopple-able admission of guilt if they do (even if the courts decide against them in THIS particular case).
I'll wait till more details come out.
Being a one-sided, no negotiation and with no "meeting of minds", most European jurisdictions won't consider this a contract -- facebook can kick any user out, but it is unlikely they could recover damages from anyone (or have standing). At least that's my impression -- though US courts, especially in states that have UCITA or similar, are more likely to consider this a contract.
edit: I've found two jurisdictions, and both of them require proof of actual economic harm for standing (among many other things). I'll wait for more specific details about Facebook's approache.....