Google does charge for Google Maps, just not for the typical user that makes up the value they're charging for. If you want to use the API for anything down to embedding a map on your site up to a full featured application like GeoGuessr, you're paying on a per-request basis once you use up the complimentary quota.

Ads certainly aren't the only way services like Maps are monetized, although luring customers to businesses is still the typical use case to pay for it. The people using it for free are the product that Google then charges businesses to leverage. Once upon a time, much of this API was free, and it's there that Google installed the premiums in this case.