I've often wondered why shows like Saturday Night Live and the Tonight Show don't charge for tickets. Economists often argue that waiting in line is an inefficient way to pay for things, so what do these television execs know that I don't? Then it hit me, waiting in line must be a signal for something that the show wants in its audience. Figured it out? Excitement! If I went to a taping of the Tonight Show, I wouldn't be very excited (until Conan takes over that is). But I would also never wait in line for tickets. By making the show "free" and forcing a line, it weeds out the moderate fans and leaves only the die hards.
UPDATE: It's also possible that they want young (and attractive) fans. Its fairly obvious that youth have a lower price on their time and are more willing to stand in line.