I can think of a few apps that do this. It is certainly possible. I think it is ethical; if someone is not participating in the open source community, they miss out on the benefits. I think most people involved do contribute in some way. If someone just wants to use Google for the benefit of ease and discoverability, then they can pay for it. You're still offering an ad-free app (presumably) and adding use value. It's perfectly reasonable to suppliment the cost of development in is way.