You can usually pick what domain you want to show up there when you buy an ad. It's common so you can use obnoxious chains of tracking links redirecting into each other and not show up as emjcd.com
I simply don't see why I should be able to buy ad that shows "google.com".
But if the ability to override the domain is truly that important, then there needs to be manual vetting of the ad buyer and the target domain. I'm sure you could automate it with signed TXT records, but I think there should still be a human in the chain to at least double-check everything.
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