Hi Joseph,
It’s difficult to guess why they did it, if I don’t know anything about the application. Maybe they were not interested in real energy deposition in the crystal, maybe it was used for some physics tests or calibration. Maybe detector was supposed to work in coincidence and something else was important (triggering, coincidences etc.).
But you have to remember, that you will have energy leakage not only from secondary electrons, but also from secondary photons (x-ray fluorescence, 511 keV from positron annihilation), and finally, you will still have primary photon leakage. In Compton scattering your primary photon will escape the crystal (with reduced energy of course), and only some part of energy will be deposited as a secondary electron. So even if you will force all electrons to deposit energy locally in the scintillator, you still have neutral particles escaping (photons). If you will force everything to deposit energy locally, then what’s the point of the simulation? 
Cheers,
Weronika