I think because Hollow Ichigo is in control that his Shikai still remains now that Zangetsu is out of Ichigo's body. You can see that Hollow Ichigo has Ichigo's sword in Shikai form, too.

My guess would be that now that Zangetsu is out of Ichigo's body (the one that trained him to Bankai), he might have lost his ability to go Bankai until he gets the old man back. I don't know about going Bankai with his Hollow self, but it could be possible.