I think search engines should be opt-in, not opt-out. Just linking to something is fine, but search engines also takes content and uses it to earn money. One example is the cards on Google where they extract content and shows it alongside ads.

Nothing else I can think of works like this. Generally if you take the work of others without being allowed and use it to make money you'll end up in trouble.

Now, for most people the trade-off is worth it, but it should not be the default. It should be a quick opt-in using robots.txt.