Why were there passengers allowed on Concorde's route proving flights? Based on this site, Concorde needed to fly 750 hours to prove its airworthiness and be used in regular service. Air France and British Airways flew two Concords (F-BTSC and G-BOAC respectively) on a total of 254 flights, with... 11180 passengers combined! How come there were passengers allowed to be on the plane when it was doing flights to get its airworthiness certificate?
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)