According to 49 U.S. code § 1154, part (b),
No part of a report of the Board, related to an accident or an investigation of an accident, may be admitted into evidence or used in a civil action for damages resulting from a matter mentioned in the report.
That is to say, an NTSB report may not be used as evidence in a civil action for damages, resulting from an aviation incident. This is re-iterated at the end of most final NTSB reports.
My question is what is the motivation and reasoning behind this legislation? While some of the elements of the report may be of a subjective nature, there is a specific section detailing factual information.
Could the motivation be to force admittance of the original evidence, rather than the NTSB report which would potentially secondary source?
A more interesting aside is also, can it be admitted into evidence if the civil action is not for damages from the incident in question? What if a report has tangential relevance in a potentially non-civil, say, criminal action?