The answer on the shielding is quite simple. When the assembly is sandwiched, the (memory) chip sits directly behind the flashtube. When the flash discharges, a fast rise current, likely in the 10's of amperes flows through the flashtube. This creates a small EMP (electromagnetic pulse) which can adversely impact any number of chips. The shield is to prevent this pulse from causing any issues with the adjacent parts. You can replicate the issue if you have a photoflash (separate, external type) from a camera. Place the window up to a circuit and pop the flash. You will see significant induced voltages in the circuit (with a scope attached) and you can easily get a micro based device to hang or reboot when the flash is discharged.
paul