When you think about it, it makes no sense that the Imperials would allow the Bounty Hunters to stand there with weapons at the ready on the bridge of their flag ship.
It wasn't "the Imperials" who allowed the bounty hunters onto the bridge of the Executor, it was Darth Vader. And the *last* thing he cared about in ESB was the safety of his officers and troops. If Bossk got trigger happy and blew Piett's head off I don't believe that Vader would have even flinched. He would have simply continued his speech about wanting the Falcon crew alive and then sent the hunters on their way.
Allowing the bounty hunters to receive their instructions in full view of the Executor's High Command was a humiliating gesture that reminded all of the officers how quickly and easily they could all be replaced, even by galactic riff-raff if need be.