Blizzard doesn't just pop out ideas. Blizzard studies reams and reams of data and comes to conclusions.
I'm guessing that they determined that the majority of their dedicated player base would be buying the expansion for the end game content regardless. Giving away the first few levels (let's face it - two or three hours of gaming) will draw in those who aren't sure about whether to buy it. Instead of $40 plus renewing a $15 subscription, they just need to renew. Low price point, more customers. Many of them will check out the new zones and leave, but many will pay to keep playing and/or to level up into the new content.