Some even have more money than they should have and you can make the case that by taking the money from them you are actually helping them. Like the young men in Saudi Arabia, who, if they didn't spend huge amounts of money and time on your game would buy a fast car instead and with some good likelihood die in it (car accidents are leading cause of death of young people in Saudi Arabia, unlike anywhere else in the world).

But there's this other group of whales that makes it immoral. There are people who are susceptible to addiction. They will spend on the game way more money they can afford just because addictive loop of the game binds them bit too much.

This is as if the games were like kind of "alcohol", mildly pleasant to lot of people, very cheap, but cheap only because industry was selling it at such a low margin that they couldn't support themselves unless fraction of people got addicted to it and used it in amounts that's unhealthy for them spending all of their money in the process. If you add to this that the industry is giving away free samples on the off chance that some new people might get addicted, you can see how that can be immoral.