Think of it this way; they are the zone's Wirld Events aka Insurgents. Just like both elsweyr zones have dragons, but instead these are portals to the Atoll. Blackwood has its specific word events as the Deadlands has theirs. The BW ones drop loot from the BW table, because that's the zone they are found in, and vice versa.
It then makes sense that only the ones from the deadlands drop deadlands-specific items, but I agree with the above in that having a note saying which zone's portal you are in would be a big help.