We are not going to decide for ourselves which of a user's logs is the best one to keep. Perhaps the log from their iPad is the "real one" and the one from their iPhone is pretty much the same flight but without the correct aircraft profile selected or something. It's the user who needs to make such decisions about what gets sent to us for "permanent" storage in the cloud, not us.
A positive opt-in would be the only way to do this properly, and I'm not sure we've seen enough interest.