There are laws in the US which require you to ship a product within a certain time or offer a refund. I agree that the 'sold out' buzz is a popular marketing technique, its one of the persuasion tools that works well, scarcity, but I think that is a side effect here.
So take a moment and imagine you are in the product manager's shoes at Google and you're tasked with forecasting how many Nexus 4's will be needed. If you guess 'high' you end up with HP's Touchpad fiasco, or Motorola's Xoom, too much product. Since you've probably borrowed the money to buy the parts to build the stuff and you have to pay it in 90 days, you need to get stuff, sell it, and then get the money to pay off the money you borrowed.
Building a million phones like this probably costs in the neighborhood of $200M - $250M. That is a pretty sizable chunk of cash to be hanging out there 'in the float' as it were. Sloshing around that much capital is like driving a fully loaded big rig truck on a wet highway, you don't want to enter any turn with too much speed, the 'bad' outcome is really bad.
So of the two options, "leaving money on the table" because someone who wanted to buy your gear couldn't, and "missing your wall street guidance" because you're carrying inventory for longer than you wanted, the former is the much better choice. At the limit its horrible but if you're going to err, and your error bars are large, you want to err on the negative side not the positive side. On the negative side you just don't make as much money as you might have wanted to, on the positive side you don't make the margins you need to make on the product.