Stern doesn't run Sirius, Stern and Imus' old boss Mel Karmazin does.
But Stern's agreement is that he controls what goes on channels 100 and 101 - and he's the main star at Sirius. Karmazin is not going to do anything to tick him off.
Stern hates Imus, so Imus would have to do some groveling to get Stern's blessing.