what about Substack's TOS?
Regarding copyright I believe that this is fair use. It lets individual readers print their subscriptions. It's indistinguishable from them using a printer at home, except that it's delivered automatically.
But yes it's a grey zone. It's making a copy of someone's copyrighted work without their permission. (Remember it was technically illegal to load songs onto your iPod.) And so that's why the book product (which brings together all posts from one publication) is for authors, since I think that strays more towards publishing rather than mere printouts.