I remember when the game was pulled from the stores. Some assholes were selling their devices with flappy bird still installed for insane amounts of money.
Sure, it is as easy as you're saying. So what's wrong with someone selling a device that is already configured with the game to people who don't want to spend the effort to do it themselves? At the end of the day, people spent money that they felt was worthwhile to get a game that they enjoy. It's the same as people selling Batocera cards on Ebay nowadays.
The game is still up in some torrents and websites. Selling old ass phones the same as a new flagship just because it has a program in it that is still available in the internet is just retarded.