People making copyright work available on the internet are granting an implied permission for search engines to index their work, because that's pursuant to the normal purposes of posting on the internet. People make work available on the internet for the purpose of allowing others to find it using search engines and view it using browsers.

However, making copyright work available on the internet does not constitute an implied permission or license to do literally anything with the posted work. People don't usually post work on the internet for the purpose of helping corporations train commercial AIs, and therefore no implied permission to do so is granted by the act of making copyright work available on the internet.