Samsung does not have a license to allow developers the actual weather information from the watch so GWS designers need to use OpenWeathermap.org.
API’s are the application programming interface and that is how an app reads information. The Openweathermap.org API and Health API are not compatible types and that is the issue for GWS.
The ones you see on the store are usually Tizen Studio developed in Native C/C++ and read the sensors. They do not use Health and often get complaints that the Health and the data shown on the watch are different. This is because Samsung has a proprietary algorithm for determining activity.
While there are 3rd party stores that basically run an emulator on the watch and the data is from the mobile phone and their own license.
Samsung Developer Program