Because we're talking about an online game demo.

Online gamers are a crowd that is savvy enough to know that a demo is a marketing tool. They're also accustomed historically to getting demos for free, because the logic is that a demo should get into as many hands as possible in order to get people playing your game and thus buying your game. And the media here is digital, easy to pass around, and generally not something that the publisher wants to restrict due to legal issues like an mp3. There is almost no cost in terms of physical media to distribute it -- no CDs to press, to sleeves to print, etc., if you were releasing it through a magazine. It's different.

Online gamers are inherent file sharers; they want to give a demo they like to their friends. They want to pony up their own bandwidth to get it out there when their friends say they don't want to wait in a line or they don't want to pay or they simply are getting low downstream from wherever they're trying to get it from.

Online gamers are virulent community members. They browse a particular set of sites and expect to see what they want on those sites.

Online gamers don't want to jump through hoops to get anything. The net is an ADD medium; they want what they want now and if you don't give it to them now they will forget about it in a few days. They also don't want to have to sign up for things, pay for things, etc. if it's a demo, which again, they're savvy enough to know is a marketing tool.

By restricting a demo using sign ups, fees, or any barriers to entry, you're actively discouraging the online crowd from doing what they do naturally: give the file out, give it to friends, and talk about it. You're basically telling them that unless they pay to get what they know is a marketing tool, they're not important enough to you to matter, and that's a slap in the face of the online gamer. And they'll pass that sentiment on to the 8 or so friends they talk to and recommend games to.