Hi. When someone uploads a GPX or KML file, we have no knowledge about where is that data coming from, so we cannot display a notice either. Same as if someone would upload a GeoJSON to GitHub, GitHub would have no idea about where was that GeoJSON sourced from. I believe users should take care about giving proper credits in the map descriptions, and if not the author should be notified about it (which we'll be addressing with an issues like feature in the future).