The intended effect of this 'stunt' was not achieved and backfired. They went for an 'opt-out' strategy where an 'opt-in' strategy is normally used. One way they could have improved upon what they did would be to only give their album for free to a select number of people. This would follow the 'scarcity' technique mentioned by Pratkanis. We can see from numerous studies that if a product is more rare its estimated value is greater on average. In a study by Verhallen and Robben (1994) they found that a product that was available to everyone was chosen less and was therefore preferred less compared to products that had limited availability.
120 female participants completed several ranking tasks on various cookbooks, however only the 'Choice' rank order is relevant here. They were given 3 random cookbooks out of a total of 18 and each participant completed forms in a 'control' period where all the books were described as equally popular, abundantly available, and easily accessible. In the participants' experimental period they were told one book had unlimited availability and that the other 2 books were 1 of four different limited availability conditions. There was limited availability due to accidental circumstances (too few books were ordered by accident), popularity, limited supply or popularity combined with limited supply.
This figure shows the results of the choice form participants had to fill out.
When looking at the U2 album example, if they had made their album limited either due to popularity (fat chance) or limited supply the album may have been more saught after and less people wanting to get rid of it. They could have achieved limited availability due to limited supply by only putting the album on a few individuals' Apple products, or it only being available for free for a select period of time. However almost 6 months after they put the album on our phones, I still have access to it and occasionally endure a song or two on shuffle. Probably about time I remove it, unless few people have it now in which case I definitely want to keep it.
Verhallen, T. M., & Robben, H. S. (1994). Scarcity and preference: An experiment on unavailability and product evaluation. Journal of Economic Psychology, 15, 315-331.