This is, of course, a tactic to entice new players.
I'm pretty sure this is only partially correct. It serves two purposes: 1) It helps people who play recruit their friends. If I'm high-level, and you don't have any characters, if Blizzard can make you get to my level faster, that means the game is more fun for both of us. 2) It helps people roll new alts. You're less locked into your character if the barrier to a new, more interesting one is lower. Blizzard has also said that the newer content is better than the old content -- the designers / artists / writers / etc. have simply gotten better at making the game. Thus, they want you to spend more time in the newer areas, as they're (generally) higher quality.
Memory fault -- brain fried