You're taking someone else's labor and profiting off it, without any credit or compensation. To add insult to injury, the person you're scraping pays money to support your traffic. It's a one sided transaction.
You can’t generalise that. Maybe I crawl to provide an annotated preview of their website, to make users of my application more likely to click the link and visit it? There are lots of ways in which crawling benefits everyone, it just requires some mutual respect.