Legends had the NR actually welcome most Imperial defectors, so long as they weren't high level war criminals, or rogues like Bevel Lemlisk who nearly gave the Hutts a Death Star.
I would argue it's actually not unreasonable for the NR to say, "If you actually had a conscience, you could have defected at any point during the actual Galactic Civil War. Defecting *after* it's said and done doesn't cut it so much."
Keep in mind that the NR is pretty much emulating the actual Allied treatment of Germany and Japan after WWII. In fact, it's actually being more lenient, since the Allies retroactively designated all German POWs as "illegal combatants" post-surrender and subsequently used them as outright slave labor. The NR's only doing that to *unrepentant* Imperials, as opposed to *all* POWs.