How do we know for sure?
There is however a less sensational alternative: Google Trends is a loss leader, to create goodwill and trust in the Google brand. Which would be interesting in it's own right; why would Google need to make such efforts to have people trust them? (nothing about helping to correct a deficit in how trustworthy google are, of course )
Still, I'd be very surprised if there'd never been corruption of Google Trends data for some kind of gain, just as could be expected from buying top positions in Google search results (I bet Google could make a real profit on that kind of shenanigans).