It seems like in the event that, for example, such buttons that paid out money exclusively to the person pushing became widespread and easily available, governments ought to band together to prevent the pressing of those buttons, and the only reason they might fail to do so would be coordination problems (or possibly the question of proving that the buttons kill people), not primarily from objections that button-pushing is OK. If they failed to do so (keeping in mind these are buttons that don't also do the charity thing) that would inevitably result in the total extermination of the human race (assuming that the buttons paid out goods with inherent value so that the collapse of society and shortage of other humans doesn't interfere with pressing them).

However I agree with your point that this is about ethics, not law.

So if the state of Georgia offers me $20,000 to execute Troy Davis; I can take the money, donate $10,000 to an efficient charity and enjoy the remaining $10,000 with a clean conscious?