It seems like Google, of all companies, can afford to let prospective developers at least try the thing they want you to pay for. I think they have the dev power to sufficiently rate limit as necessary.
Frankly the way Google has been advertising their AI offerings shows they don't care about the consumer market. I hear great things about their LLMs, but for some reason I never try them, and most people haven't either.
This is a small group within Google doing experiments, they are probably trying a different approach after the MusicFX app experiencing heavy usage and needing to limit users. They didn't have a way to supply your own API key for that one
You can easily try Gemini for free in a number of other places
Frankly the way Google has been advertising their AI offerings shows they don't care about the consumer market. I hear great things about their LLMs, but for some reason I never try them, and most people haven't either.