As andybak says, almost certainly privacy. It's very easy for an unsophisticated user to share photos that, for example, effectively show where they live (or where someone else lives). I believe Facebook strips geo data as well. (A service like Flickr does not although I assume there is a switch somewhere that tells it to.)
Well it's being done through the API, which is presumably OAuth'd under your name, but again, this API is generally used by other sites to interact with your photos library, and it's very easy for someone to just approve the OAuth permissions for sharing photos, not realizing they are also implicitly sharing their location.
One scenario is you're at a party at someone's house. If you post a captioned photo from the party that's geotagged, you're identifying where they live.
