OK, thanks, I understand now, many thanks.
In current implementation, there's no reason why a developer couldn't add any URL obtained via Windy Widget UI (which doesn't explicitly display any prohibition), in any IFrame on any web application.
To avoid such a stuff, I guess Windy would be forced to continuously track calls to such an URL, which should be boring at the end...
Of course adding this info in API docs should be doable and useful.
Changing the API is more difficult: it would require all API clients to change the queries, then anyway values format in queries is just a convention.