Board Thread:Lore Discussion/@comment-26316533-20150518132454/@comment-27253250-20160215173422

IIRC Haskill explains it- it's an invitation, which bears no inherent threat. Daedra don't really come through it, and it's not nearly as a deadly as going through an oblivion gate.

Better explanation: would you rather have the DLC become inaccessible just because you went through the trouble of finishing the main questline?