This is a huge topic of debate, but the consensus from "experts" seems to be that if it's public data that's not behind some auth, then it's fair. By the way, this is what web crawlers, like Google's crawler, does.

See: https://www.quora.com/Is-website-scraping-legal-and-ethical

But I don't necessarily want to go down this rabbit hole here, as it detracts from the interesting technical issues outlined in the article. If you haven't read the entire article, I recommend that you do, because they had to overcome many interesting technical challenges while working on this.