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They are business, not some random visitor. Google scrapes websites all the time, provide Adsense, a way to earn money.

Adsense pays you money for showing ads to human visitors, you don't get paid for allowing their crawler.

You are letting them to crawl your site and earn money, they came up with a genius idea to pay you. Simple

Google's crawler makes your page show up in their search results and gives you visitors.