You weren't sucker-punched. You shot yourself in the foot. And the big reason you are so ticked off is that deep down, you know that you are doing it again by blaming the players for your mistake.
Take your medicine. If you vent you spleen by sucker-punching teh players they will be very rightly unhappy.
Wraithdrit said:
Also, I awarded XP for bypassing the dangerous encounter, so what do I give for going back?
Nothing. The PCs have already been adequately rewarded for overcoming this obstacle. The shadow dragon is not an obstacle in their story goal any more, and they don't get any experience for picking an unnecessary fight. Just apply your usual standards for when the PCs go off on a side-track risking their lives for gold. If the survivors end up with treasure, that should be reward enough.
If you run it any other way your PCs will first avoid and then unnecessarily slaughter everything, for double experience.