What's more, Michael basically gave Rocco no choice. The sequence of dialog is important:
After humiliating Tom about the mistress and the offer from the House and Hotels (note Neri smirking, which tells me he was the source of that info), Michael turns to Roth's arrival and says, "I want it [the plane] met." Tom responds with his "impossible...do you want to wipe out everyone?" speech. Michael smugly replies, "Y'know Tom, you surprise me...anyone can be killed." He instantly turns and says, "Rocco?" After all that put-down of Tom, Rocco really didn't have the option of saying, "Uh, Mike, uh, I agree with Tom, it's impossible."