The point was that they would he excuses from the readers pov, Shinobu pov, even if it came to light with her friends that Shinobu wasn't actually a bad person, the next question they would obviously have is "well why didn't you say something when we were bad mouthing him?" If Sakura just says that she was too afraid or insecure to, would they not see that as excuses? This world doesn't operate through Sakura's pov and people do just act solely on their pov, if they do, they are probably either extremely antisocial or irresponsible.