I don't blame him either. Random British guy writes to you in French and ask for your source code. Sure I'd be skeptical too (although personally I err on the side of releasing all my code).

The problem is that suppose it is a bug and suppose the code is being kept behind so they can find the bug themselves. A huge amount of time and money is wasted whereas a programmer could look at the code and compare with the paper pretty quickly.

Also in the case of high-energy physics its not exactly easy to duplicate experiments because of the eye-watering cost of the equipment.