Google et al. run crawlers primarily to populate their search engines. This is a net positive for those whose sites get scraped, because when they appear in a search engine they get more traffic, more page views, more ad revenue.

This is not necessarily true. Google’s instant answers are designed to use the content from websites to answer searcher’s questions without actually leading them to the website. Whether you’re trying to find the definition for the word, the year a movie came out, or a recipe, Google will take the information they’ve scraped from a website and present it on their page with a link to the website. Their hope is that the information will be useful enough that the searcher never needs to leave the search engine.

This might be useful for searchers, but it doesn’t help the sites much. This is one of the reasons news companies attempted to take action against Google a few years ago. I think a search engine should provide some useful utilities, but not try to replace the sites they’re ostensibly attempting to connect users to. Not all search engines are like this, but Google is.