because the rest of the world follows the same idiotic flight rules that we enforce here (no outside carriers flying between cities). Air France is one of the largest airlines in the world, Delta's partnership with them gives them access to places they couldn't ordinarily reach.
Most major U.S. airlines partner with European carriers:
- United/Luftansa/SAS
- American/British Airways
- Northwest/KLM
- Delta/Air France
There have been efforts to modernize air travel rules, but I can't imagine that given the current situation we're ready to allow foriegn carriers to fly between U.S. cities.