I believe this stems from a time before Azure Resource Manager update which allowed more flexible setting of IP ranges. I don't think there is any specific reason why we couldn't allow users to specify an existing subnet range anymore, just historical baggage and not having the time to come back and clean this one up.
Thanks for the bug report, we'll try to get it into the queue for the next major release.
I believe this stems from a time before Azure Resource Manager update which allowed more flexible setting of IP ranges. I don't think there is any specific reason why we couldn't allow users to specify an existing subnet range anymore, just historical baggage and not having the time to come back and clean this one up.
Thanks for the bug report, we'll try to get it into the queue for the next major release.