Google Maps's "API key" is not really a secret. It's used only to identify your application and to generate an iframe that's only allowed to be used on your website. It's bulletproof enough not to be considered as a secret as it can't really be used to impersonate your app if leaked.

> it can't really be used to impersonate your app if leaked

It actually can, I could create an app called com.yourbusiness.someapp, install the app directly without signing it, and use yourbusiness's API key.

For the JavaScript embed APIs I could create a fake root cert, fake DNS, fake HTTP cert signed by the fake root cert, serve https://yourbusiness.com/ from my in-house server to my own laptop, and then issue queries to the Google API using yourbusiness.com as the referrer. (Or hell, just create a Chromium fork that serves up fake referrer headers.)

It's just that it's not big enough of a threat that it has become an issue.