From what I understand, it's just on the weeks that the new DLC is in the Nightfall and Weekly Heroic that people would be locked out if they did not buy the DLC. You should be able to do the Nightfall and Weekly next week. I think if they made a separate Nightfall and Weekly, people would then be complaining that people who bought the DLC get to run two Nightfall's and Weekly's instead of just one. I can understand why they did it the way they did.