AF/FB has a lot of leeway written in its T&C.

The basic motivation is that they do not like people who never fly AF KL DL to open an account and transfer miles from cc for the purpose of booking an award. My guess is that they feel it defeats the purpose of an airline "loyalty" program. That is besides any possible fraud issues.
It seems from posts on FT, that they periodically target such accounts and ask for "verification". They make you go through painful hoops, such as going to an airport to verify identity. I don't know how frequent such a request for identification is.