The BEA report is a legal document
Without worrying about international law and taking into account only the French laws we can say that:
The French state has requested (through the Minister of Transport) to the BEA to conduct the accident investigation
BEA is an agency of the Ministry of Transport
This report will be produced at trial (how could it be otherwise, since this report accurately describes the accident and provide related documents)
This is a common thing (see last trial dated .. the Concorde)
This process is also an opportunity to examine whether the BEA has followed the law to conduct its investigation .. because the state is obliged to respect the laws
As I write before ... wait some years for the trial ...