It's Google asking for an API key to a google service. "Simplicity" here would just be them letting you run the demo. Like yeah, all of the "authentication, rate limiting, cost capping, scaling" is done by Google, it's a Google page running a Google service.

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.

Google can afford to run this model (a bigger one, actually) in their search results when I don't need or want it. Why shouldn't they run this tiny demo backed by their least expensive model? The total cost of running the project for it's entire lifespan is probably less than the value of two weeks of a single engineer's time.