Before we can purchase or use any apps, libraries, addins, etc. our legal wants to review a EULA or software license agreement. Does one exist for ExifTool?
See the ExifTool License post (https://exiftool.org/forum/index.php?topic=4002.0) in the Developer's subforum.
As an example, Flickr uses exiftool on the backend (https://code.flickr.net/2012/06/01/parsing-exif-client-side-using-javascript-2/).