Other mistake: In the scene after De Niro's car explodes, they show him being put in the ambulance feet first. Patients are always put in head first.
Other mistake: When the man goes into the casino and takes a suitcase full of money to Kansas, he is parking in a supermarket. Later in the movie, after a plane lands on the golf course the same shot of the car in the supermarket is repeated. Check for the same parked cars: a blue VW beetle and a red car.
Other mistake: When Ace meets with Lester in the diner, Lester looks around at the guys guarding the exits. The camera pans around to show his perspective, looking first at the door directly behind him, and then the door to his right. Except the camera swings left for both shots, meaning Lester would've had to crane his neck around 270° to see the right-hand door while looking to the left.
Answer: It's also possible that the "bosses" would be concerned that a rift between Nicky and Sam could bring public (police) attention to the criminal activities going on behind the scenes. These are, after all, two very egotistical individuals.