When the AAIB, using their expertise, discover that a pilot was at fault, or indeed, was not at fault, why should that be hidden from the police officers investigating an incident?
It is not the AAIB's task to apportion blame. Their remit from Annex 13 is to discover why an accident occurred and make recommendations to prevent one happening again.
The Police have a completely different brief. As far as they are concerned, if there's a death, there's a crime to be investigated. For example, they no longer use the tern 'road traffic accident', preferring instead 'road traffic collision' as the word accident infers that no-one is to blame.