I don't see your concern as an issue. The game has to be fun. If it's fun, people either don't care about the side-effects, or are only turned off if those side-effects are extremely detrimental: look at World of Warcraft. It's really just that simple

Even then, you'd be surprised how much people like the idea of, you know, helping someone else out. Maybe you are pointing out that most of us are so gree-, err..., industrious, that if we imagine we are contributing value to someone, we feel like we shouldn't be giving away that value for free. Thus, playing such a game just makes us feel cheated.