Whats the point of PAX exactly? Wouldnt the point of having a booth be to advertise your product? Kindof dumb to advertise to only a small number of people and not allow them to record; resulting in free mass advertisement.
They might as well build a wall around the booth and say "No one is allowed to see whats on these screens.... no one!"
But really, I have no idea what these expos are.
Have you ever seen Men In Black. The speech that Agent K gave J. "A person is smart. People are dumb, panicky dangerous animals and you know it."
The build that Gearbox currently has is probably rather rough. Its an early example of gameplay. That fact is easy to explain to a few people in a small area, but even if you have it plastered at the beginning of the footage before releasing it to the masses, people will still point out all the flaws and immediately come to the conclusion that the final product will be bad.
It sucks for the fans and the people who can't attend the expo, but in the end its Sega protecting the image of the game. Many companies do this. In fact if I remember correctly most of the games at PAX were sort of hidden away. Mystery is sometimes more intriguing too I guess.