Why wouldn't it be? I know we've all got accustomed to the idea that preventing a billionaire from extorting money gets you in trouble, and it usually does, but it's not illegal.
>Why wouldn't it be?
Well, for one, because xcancel was forced to suspend its service.
There's legal as in "what the law says", and legal as in "what the judge will order you to do", and the latter sense is not as clear-cut as we'd want it to be.
Is it legal to make the whole world addicted to your product? Litigation, in the same way, is a ticking time bomb for meta worldwide.
Xcancel was accused of using the twitter API. You don't have to use it. You can have a machine vision model process screenshots. You can also process the Dom. You don't offer a freely usable service or software to the public, only the final result. That final result is opinionated according to the very public accounts you want and does not offer broad scope access to accounts. You do not let the public control it nor provide a product to browse to random or posts about anything like nitter. Or you can process the content into your own news account with ai which is what most news accounts do anyways and then it becomes yours.
xcancel wasn't actually forced to suspend its service. It got two increasingly threatening C&Ds, but nothing actually happened to it, and the creator took it offline in an abundance of caution.