Why are manufacturers assumed to be responsible in case of a crash?
I'll assume you're talking about an English common law jurisdiction (UK, US, Can, Aus, NZ etc.).  In those places it's always &quot;innocent until proven guilty&quot;, the key thing being what &quot;proven&quot; means.
You have to separate criminal liability, breaking of a law in the criminal code, from civil liability (a civil wrong, or &quot;tort&quot; in English common law).  The criminal liability standard of evidence is &quot;guilty beyond a reasonable doubt&quot; at trial, and tort liability standard is &quot;guilty based on preponderance of evidence&quot; at trial (that is, more likely than not in the assessment of a trial jury (in the US) or a judge).
So a manufacturer or an airline is only exposed to criminal liability where an incident involved the violation of a criminal statute. You can't throw the &quot;company&quot; in jail, so an individual has to be on the hook and so the authorities would have to identify a criminal code violation committed by an officer or employee of the company, and it would be that employee or officer that would be going to jail (for something like violation of a criminal negligence statute).  And in that case the standard is guilty beyond a reasonable doubt.
In the case of civil liability, the corporation itself, or potentially its officers personally, are on the hook, the penalties being solely financial (employees are normally shielded from civil liability in a limited liability corporation) and the &quot;preponderance of evidence&quot; standard of guilt applies.
The thing is, both of these conditions require a court trial.  Either government prosecutors go after individuals (officers of the company or employees) for criminal violations that include fines and/or jail time for those convicted of criminal acts, or non-government individuals that have been harmed (families of passengers killed for example) go after the corporation in civil court, and the corporation is only guilty of a tort (civil wrong) if a judge or jury finds them guilty.
So in both situations it's &quot;innocent until proven guilty&quot;; the difference is in who the accuser is and what the standard of evidence is.
Now, outside of the Anglosphere, all bets are off, and you may very well find places where guilt is assumed and company has to prove its innocence to get off the hook for either criminal or civil liability.