How can a web browser or even native OS determine if a user's wifi network is metered? I agree it would be nice in theory if there was some kind of "dollars per MB" API, but I don't see how web browsers could implement that in practice.

I work on a podcast app. When people are downloading hundreds of MB or more daily, they emphatically want control over when downloading takes place. In practice, almost all of them are satisfied with a binary choice between cellular or not. Those who do sometimes use expensive or slow wifi networks can blacklist them, which is also what network info APIs support.

There are also other applications such as metering apps, utilities that trigger actions upon network changes. Some very popular Android apps (e.g. Tasker, IFTTT) rely on network info for that; web falls further behind if web apps can't access that data.