> HTTP/HTML is also an API, just a bit more cumbersome to parse than JSON.

No it's not, HTML can change anytime. It is meant for human consumption. "API" has the word "Interface" in it because there is a well defined interface to a service, and is documented, and is meant for machine use predominantly.

> API key

While I agree with you, I also think it is important to be able to disable applications that might be misusing the API quickly.

> And of course you need an "API key". It is public data, just make it public

They will want some kind of mechanism to cut off abusive traffic, and API keys are a much better choice than IP blocks.

It's easier to limit who misbehave that way