Why not? - Law 43A1 (b) Dummy may not call attention to an irregularity during play. © Dummy must not participate in the play, nor may he communicate anything about the play to declarer. Introduction - for clarification "penalised),”shall” do (a violation will incur a penalty more often than not) “must” do (the strongest word, a serious matter indeed). Again “must not” is the strongest prohibition, “shall not” is strong but “may not” is stronger – just short of “must not”. I mean it doesn't happen often but dummy could be a better player than declarer and surmise that declarer is about to take a losing finesse when the drop might work . . .