The direct answer is their clients play extra nice with their backend.

Specifically all optimize caching.

The indirect answer is for everyone using third party tools to play about there are 10x using it to spam or malicious use cases hammering their backend far cheaper than if it was by API.

These people are the false positives in this situation, but whether Google or Claude care is unlikely. They're happy to ban you and expect you to sign up for the API.

This has always been a worry when you use a service like Google.