Yes, because the author didn't make its contents available for free. Websites can do the same, and many do. They could even disallow other sites from crawling t

Yes, because the author didn't make its contents available for free. Websites can do the same, and many do. They could even disallow other sites from crawling them and processing their contents through the (mostly) respected robots.txt. But publishing something for free, not forbidding crawling through well-understood mechanisms, and then demanding payment is ridiculous.