Many revenue-based websites tried to have it both ways with web crawlers wherein they wanted to block automated access or repeat viewers while letting first time viewers get a free taste. Others have noted that basically Google gets a free pass for all the traffic it brings in but everyone else has to respect robots declarations.
It seems like a no brainer - if your web server is configured to reply to GET requests with a 200 status and some content then they get to do pretty much whatever they want with it.
Don't want to give access to everyone? Stop sending your content for free and get them to agree to some contract and authorize/license their access to your stuff.