If Mastodon gets content licensing right, you'll still be able to ignore it and go ahead and crawl data when the license forbids, it scratches your itch and you're ethically challenged. But then if you do anything with that data in public you're going to get legal nastygrams. That may not even stop you, but it will drive up the cost of your lack of ethics.
Ask any security pro. You can't ever stop all the attackers. All you can do is make it more and more expensive to do bad stuff, and eventually most of them won't have a strong enough incentive to pay the price.
There are plenty of people on Mastodon - the vast majority is my bet - who, when there's a choice of content licenses, will cheerfully say "make it public", and then there will be excellent full-text search.