How do you combine microtransactions with the need to be indexed by search engines?

Microtransactions solve the issue of bad bots, and possibly websites monetization. But then do you want to give free pass to search engine crawlers? The big ones will be strong enough to refuse to crawl your site if you don't. The small ones will be financially unable to crawl if you don't. If you allow them all, you're back to step 1. If you allow only one or a few, you basically freeze search engine innovation.