Perhaps it would make more sense to say people shouldn't spend money they don't have on things they don't need. However, even that advice isn't useful for the poor because they already follow that rule. It's also not needed for the wealthy, because they have more than enough money to buy what they want. It's only useful to people in the middle who have enough extra income that they can afford to pay their cards' minimum payments after spending on things they don't need but feel like having, and mistake that for meaning they can afford to buy those things.