Um... why? Does Uncle George own the rights to twirl a light stick around? 
A Stick? No
A LightSaber? No
A Lightsaber Twirl at an official 501st event? Yes
In Short, the official Reason:
The 501st, and The Rebel Legion, are an official organization that has limited use
of a copyrighted product, image and such. Our use only continues as long
as LFL grants us the say so. They could, at one swoop, destroy everything
we have, to include events, websites, products, ect ect. LFL has made
the decision based upon that at an official 501st event we are a representation
of LFL themselves. In which case if anything ever were to happen, such as an
injury, during a lightsaber duel LFL would be liable.
Even shorter, because they said so. End of story.