The calculation (for Google and for the advertisers on Google) is not "does this have a positive RoI as compared to zero?" but rather "does this have a more positive RoI than all my other alternatives?"
When the risk-free rate increases, the RoI hurdle for any investment also increases. That's how economic stimulus by lowering interest rates works, by creating incentive to "try things" (by removing the incentive to "park your money").
Advertisers on Google now bid on ads based on an environment where they are more conservative (because they have better alternative investments), where their input costs have increased (labor, raw materials, energy), where some industries (like mortgage refinancing) have been dramatically curtailed directly by the change in rates, and for some companies, buying back their own bonds is more attractive than advertising on Google with a portion of their spend.
Google can look out on the horizon and conclude "OK, we had a really great business when rates were near zero; now we have a pretty damn good business with rates several points higher than that, but it's definitely not as good as it was, so we need to tweak our frugality dial in response."