My theory is that a lot of it is back-rationalization. People just have a gut reaction against it and search for any reason they can find to argue that it's bad.
The main reason it's different for game developers is that in most instances, in-app purchases will undermine the integrity of the game more significantly than for other kinds of apps. Indeed that is the main point of the OP ("There is no game here.")
If you add a rule to chess where you can pay cash to put a taken piece back on the board, you don't have chess anymore. Additionally, consider that videogames are often escapist entertainment that people play specifically to get away from real-life financial pressures or wealth-based status sorting.
The main reason it's different for game developers is that in most instances, in-app purchases will undermine the integrity of the game more significantly than for other kinds of apps. Indeed that is the main point of the OP ("There is no game here.")
If you add a rule to chess where you can pay cash to put a taken piece back on the board, you don't have chess anymore. Additionally, consider that videogames are often escapist entertainment that people play specifically to get away from real-life financial pressures or wealth-based status sorting.