That's exactly what I'd call Beta testing. First Bounties had high rewards, everyone was running them by the hundreds a day, now Nephalem Rifts are being run by the dozens every day. And now both of them have had extensive testing with players doing the same thing over and over for a great amount of hours!
Trust me, the reward part is easy. But players first had to try that feature over a looong period of time so that its mechanics are very well explored.