you make a valid argument, they are one of the most security conscious companies out there. But I still believe its a security risk.
Lets say a user is generally connected to a network. The network becomes compromised so they quickly disconnect their phone. The next morning when returning to work they network may or may not still be compromised, the phone they thought was taken off of the network has suddenly connected itself back to they compromised network.
i'm sure people smarter than I will be able to think of more security risks than I was capable of thinking of in the 13 seconds I thought about this situation. But i'm sure apple thought about this as well.