The guilters' argument is essentially that all police and prosecutors are always honest and ethical - that they would never engage in misconduct by virtue of their office, and so the ones in Italy involved in the Knox - Sollecito case were totally honest and ethical, and were by virtue of their office had not engaged in misconduct in the Knox - Sollecito case.
Yet there are cases that clearly show that not all police and prosecutors are honest and ethical and do not commit misconduct. Thus, one cannot logically infer that the Italian police and prosecutors were honest and ethical and did not commit misconduct.
For example, a number of Italian police were dishonest and unethical, committing official misconduct including violations of Italian law and human rights under the European Convention during the G8 meeting in Genoa. Here's a media article about the latest penalty imposed on those officers; I don't know if this is a final judgment.
G8 Genova: Bolzaneto, 26 condannati a pagare danni allo Stato per 6 milioni
Per risarcimenti sborsati alle vittime degli abusi durante il G8 di Genova nel 2001
Sei milioni di euro per i danni causati allo Stato in seguito ai risarcimenti pagati a chi subì gli abusi nella caserma di Bolzaneto durante il G8 di Genova nel 2001. Lo hanno stabilito i giudici della Corte dei conti di Genova che hanno condannato 26 persone, tra personale medico-sanitario, appartenenti della polizia, carabinieri e polizia penitenziaria. Tra questi, come anticipato nell'edizione locale di Repubblica, anche Alfonso Sabella, all'epoca dei fatti capo dell'Ispettorato del Dipartimento dell'amministrazione penitenziaria (Dap) e il generale Oronzo Doria, ex capo area della Liguria degli agenti di polizia penitenziaria.
....
G8 Genoa: Bolzaneto, 26 sentenced to pay damages to the State for 6 million
For compensation paid out to victims of abuse during the G8 in Genoa in 2001
Six million euros for damages caused to the State following the compensation paid to those who suffered the abuse in the Bolzaneto barracks during the G8 in Genoa in 2001. The judges of the Court of Auditors of Genoa who sentenced 26 people, among medical personnel, members of the police, carabinieri and penitentiary police. Among them, as anticipated in the local edition of the Republic, also Alfonso Sabella, at the time of the facts head of the Inspectorate of the Department of Prison Administration (DAP) and General Oronzo Doria, former head of Liguria area of the prison police officers .
....
Source:
http://www.ansa.it/liguria/notizie/...to-_b1818cec-c04f-4cb4-ba1e-5e720cc8b611.html
Translation by Google translate
Here's another example, from Spain, contradicting the guilters' claim:
Five men, including a police officer for the paramilitary Guardia Civil, were convicted of jointly (as a "wolf pack") conducting a sexual assault on an 18 year old woman, and the police officer was fined an extra €900 for stealing her phone.
Source:
http://www.bbc.com/news/world-europe-43935380