Frequent flyer miles aren't free. One way or another, you pay for them.
Typically, the price of the miles is built into the price of the product. Which leads to the following side note: Since you've already paid for the miles, you're not getting full value for your dollar if you opt not to earn them.
Sometimes, though, miles are awarded in exchange for one's time rather than one's money. An example is the new Bose promotion.
For the next two weeks, members of American's AAdvantage program can earn 150 miles by watching a two-minute online video extolling the virtues of the new Bose noise-canceling headphones.
That's 75 miles a minute, 4,500 miles an hour. Or to put it differently, it would take just over 5.5 hours to earn a free domestic award ticket. (That's purely hypothetical—the terms and conditions stipulate that "AAdvantage miles can only be earned once per AAdvantage account.")
As a point of comparison, there's also a 350-mile bonus on offer for sitting through an in-store demonstration of the headphones. Given the likely time and expense required to visit a Bose dealer, that's not very appealing.
And finally, there's a 1,500-mile bonus for actually purchasing the headphones. At $299.95, that amounts to earning five miles per dollar. That's a decent payout. And if you're going to buy the headphones anyway, it would be economically irrational not to take the miles in the bargain. After all, you'll pay for them, one way or the other.