Well the TK website doesn't let you pick (you can't even click on anything), no luck using the app but get "paid seat selection is not available for this flight" for the TK flights. Someone has suggested hitting them on Twitter which I'm attempting to do.

Those in the TK forum are saying "Turkish is great in the air and terrible on the ground", I definitely agree!

PS One Turkish agent tried to convince me that she couldn't assign seats because they have changed the flight, and United will instead fly FRA-IST instead of the Turkish plane that was originally scheduled to do it. Does she not know how much we know about how things work???!!!