IMO, there is no reason why a website owner would not be entitled into knowing which keyword has brought the buyer onto her/his website.
Privacy has nothing to do with it. The seller already knows the name and address and credit card info from the purchaser.
If the purchase has not been made, there are no names or any other personal information anyway.
When people get greeted in the store they're often asked about if they were looking for something in particular. Then, the potential buyer could say "just browsing" or "pants"
Ah, anyway... Google... They have too much freedom compared to how much of everything they actually control. Is that called a monopoly?