I've just been informed by someone at The Avocado that they ALWAYS give a 30 day free trial instead of the industry standard 7. Those evil, manipulative bastards. I guess they're putting that present icon between the DVD and bell icons to fool people. Sorry.
Yep, this was originally done to convince people that a DVD rental service could actually work. The thirty days nearly broke the founders initially because Netflix became popular so quickly--they tried to sell to Blockbuster who didn't want to buy them out. I guess the 30 days just became a permanent fixture of their brand.