I have read studies which indicate that people will often not respond in such situation, it seems to be a human thing not an American. Even if such things are in the books, I suspect that hey are not enforced except selectively.
All suggestions are that the postal police themselves were extremely reluctant to respond. They should have been charged with a crime by the way you are arguing.
Knox claims she waited hours before giving alarm to the police and to Raffaele. She lied to Filomena about the place where she was. She did not answer Filomena's calls. She did not tell the truth about calling Metedith. She rang Meredith only for five and three seconds, and didn't even call her on her second phone eve after she had just told Filomena she would do that. She said she had become worried and told Filomena she would call for Raffaele's help, but in fact she didn't tell him and instead she waited further, then she said she was not sure whether there was something to worry about.
Those are the situations where you would see if someone is a reliable person or not. I wouldn't be happy of having a friend who is incapable to do something consistent with being concerned for a person.
By in fact Knox's story is also not credible.
Then when Knox goes to Sollecito's house, and when they come back home again, Amanda shows concern for carrying a useless mop to clean up only a little of water that was there from the day before (?didn't dry?) even if there were rags of all needed kinds in Sollecito's apartment.
When she enters her own apartment - she tells - she was only worried to check if someone had stolen her computer, even if she had been on her room for a while to change herself and even if she stated that she had come to a conclusion that nothing had been stolen from the house since Filomena's laptop was there.
Sollecito says in advance that nothing has been stolen and repeats it, Knox contradicts herself and Filomena about the locked door.
If that was not enough, she tells Filomena she would call the police immediately, but in fact they only call the police at 12:54.
These are only son of the things one notices just from the start. Only some of them and only about how the thing starts.
Any normal person with no agenda would notice those contradictions.