recipients of grants are heavily restrictedThey are restricted in trivial ways that are easy for a bureaucracy to mechanically enforce, as is true of employees at every institution.
What I meant by accountability is deeper: people are not accountable for the quality or utility of their work, hence the endless tidal wave of corrupt and deceptive research that pours out of government funded 'science' every day. These researchers probably filled out their expenses paperwork correctly but the final resulting paper was an exercise in circular reasoning, or the data tables were made up, or it was P-hacked or whatever. And nobody in government cares or even notices, because nobody is held accountable for the quality of the outputs.
Whilst DeepMind is not especially interested in profit it's true, and is just doing basic research, Google itself is an excellent example of how to seamlessly integrate fundamental research with actual application of that research. That's what profit motivated research looks like: just this endless stream of innovative tech being deployed into real products that are used by lots of people, without much drama.
We have come to take this feat so much for granted that you're actually asking if someone working on ads is leaving the world better off. Yes, it does. Google ads are clicked on all the time because they are useful to people who are in the market to buy something. Those ads are at the center of an enormous and very high tech economic engine that powers staggering levels of wealth creation. If I understand correctly, a lot of academic papers are actually never cited by anyone - a researcher who optimises search ads by just 1% will have a positive impact on the world orders of magnitude greater than that.