ive had this happen on my 105 5600 crankset. same "mechanics"/tech as the dura ace 7800 (or any 105 and up shimano crankset).
i installed it in a rush, and did not double check my work. i rode probably 40-50 miles before i looked down at my crank arm dragging, still clipped in.
cant say for sure, but id put my money on mechanic incompetence. if they are going to install a crankset, or any component on your bike, they should be aware of obvious physical damages, and warn you ahead of time of possible danger.