Apple takes a financial hit on
I don't think they underestimated demand at all. They knew exactly what demand was, and purposely undersupplied the market. They in fact predicted this when they said they weren't going to release first-weekend sales figures, breaking with years of tradition. In their
statement to NBC, Apple specifically said:
"...we know before taking the first customer pre-order that we will sell out of iPhone 7."
Apple knew exactly how many iPhones it was going to sell - all the iPhones it supplied. So this isn't a case of Apple being surprised by the demand. The fact that they won't release numbers means they know the number supplied was low.
HOWEVER...It also isn't the case that Apple is artificially holding back supply to gin up hype or create the illusion of hot demand. Ideally I'm sure Apple would like to have an iPhone available for every person that wants one, and this "fake demand" theory is both silly and bad business.
The truth is Apple got bit hard with the 6S and had to draw down inventory because of a supply glut - the channel was stuffed with 6S inventory that nobody wanted to buy. That costs Apple a lot of money.
The problem is that demand is always highest at initial release, but you can't just spin up factories/machinery/workers/etc. to fulfill that demand for a few weeks and then make them disappear when the demand goes away in a month. You need to set up your supply chain for how many devices you're going to make over the course of the entire year. Apple is simply being very careful to avoid the oversupply issue they ran into with the 6S. They figure that Apple loyalists will just wait until October or November anyway, so why take the risk? It's not like they're going to lose those sales to a competitor - people who pre-order are generally die-hard fans who won't just buy a Samsung if their iPhone is delayed by 3-5 weeks.