User blog comment:Bobbs3k/Can Ichigo's New Bankai be stolen?/@comment-6963992-20130630205001

Ichigo was using a fraction of his shinigami powers not his Quincy powers and he is now using his Quincy powers in his second sword regardless so it's probably because of his spiritual power. Ishida could steal it for plots sake, but I'm just guessing so who knows.