According to company president Andy O'Neil, Bluepoint reverse engineers the games, rather than collecting the original source files and recompiling. "The problem with source asset drops is that you don't know if it's the final shipped data," O'Neil said. "People will go and make last-minute changes."
Of course, Bluepoint is careful to keep the games running as-is, even when that keeps bugs found in the original versions. Some bugs were fixed for the PAL Shadow of the Colossus update --- "But it was a deliberate decision: this can break the game," O'Neil explained. "But we won't change anything without talking to the original developers."