Why were there passengers allowed on Concorde's route proving flights?
Bear in mind that these tests weren't to prove the safety of the aircraft per se, just to show that it would work as an airliner. It had to be loaded up with an appropriate complement of passengers and luggage to prove this. 

However proving flights may not generate revenue from passengers - they are either selected through an application process or chosen at random, and take part in the proceedings free of charge.

CAA AIRWORTHINESS INSPECTOR‘S HANDBOOK (Page 2, point 3)