Behavior Consistency. I recently learned about this phenomenon. People make decisions on an emotional level and then logically support them afterward. In fact, there are studies where people who have had their brain damaged and no longer process emotion have a really hard time, or simply can't, make decisions.

Someone has to emotionally decide to put X amount of hard earned money on something before hand. The rationalization of this affects the overall experience. When an item is free their commitment to the app was zero.

There was a fascinating study about this.

Festinger, L., & Carlsmith, J. M. (1959). Cognitive consequences of forced compliance. The Journal of Abnormal and Social Psychology, 58(2), 203.

Where participants were given a task to convince other people to perform a long and boring task. One group was given $1 to convince people while another group was given $20. They found the $1 dollar convincers actually believed what they were saying where as the $20 group shrugged off their actions to convince people to perform the boring task as, "I did it for the money." $1 was too low to rationalize away.