The Black Family Reunion was apparently scheduled before the 9/12 Tea Party, so they already had the permit for the Mall.
In all fairness, on one expected so many people to show up. I think I read that the Tea Party organizers expected a maximum of 20,000.
As it was...the numbers were there and spread out around the other event. The picture looking from the large intersection all the way to the Capitol (over a mile distant) with the intersection and the 8 lane road leading to the capitol absolutel full of people (and it remained full of people moving towards the Capitol for almost 3 hours!) tells the tale.