Why did Marjane's grandmother yell at her?

The reason behind Marjane's grandmother yelling at her is very simple and justified. On pages 285-286 Marjane sees guardians of the revolution coming for a raid and realizes she is breaking their rules by wearing makeup. She doesn't want to get in trouble so she finds an innocent man on the street and tells the guardians of the revolution that he was being indecent to her. They take that man away and probably hurt him badly, but Marjane is safe and goes along with her life. She tells her grandmother about this thinking what she did was hilarious and her grandmother becomes furious because Marjane's uncle and grandfather fought to defend innocent people and gave their lives for it. It is obvious that it is certainly justified for anyone, especially her grandmother to be upset with her because what she did was cowardly and horrible. Luckily she realizes that what she did was bad and she promised herself she would never do anything to make her grandmother yell at her again.