So far, I do not find the related link of licensing requirements or any restrictions for this service.
From the link: https://www.microsoft.com/en-us/microsoft-365/blog/2013/04/10/office-web-viewer-view-office-documents-in-a-browser/, It's a service that creates Office Web Viewer links. We can use it in website or blog (e.g., recipes, photo slide show, a menu, or a budget template) to let users read files online without downloading.
In my opinion, this service is free, as it does not allow people edit the Office files. Like no activated Office apps, they in view only mode.
Besides, it seems that no related development documentation or guidelines.