They probably felt that Ernie was just another employee of Frank, just like they had been until Tony had Manny shoot him. The scene shows the childlike unpredictability of criminals like Tony and Manny. After a tense, angry killing, they're friendly and chummy again, almost human. They're living on a different level of reality and don't know it. Ernie does, however, that's why he nervously took that swig of whisky after the tingly moment had passed. Good final touch by scriptwriter Stone in that sequence.