[hayksaakian]

Given for example the geocoding API, which is js based and requires no API key, I don't see how they would know who to charge.

[untog]

As stated by the docs:

Use of the Google Geocoding API is subject to a query limit of 2,500 geolocation requests per day

if you're using the server endpoint, which perhaps checks what server the request is coming from- I have no idea. The JS, client-side version definitely requires a key.