It’s supposedly bad to exploit poor people by paying them as little as you can get away with in trade.
Onlookers who don’t offer the needy anything condemn those who offer some non altruistic benefit through trade because it isn’t enough. It’s interesting that we see this as a fault with the person trading, rather than with everyone. Very few people think they themselves are morally obliged to pay the poor more. I’ve discussed before how misguided this is if we care about the wellbeing of the poor person. But why do people feel this way? Here are some reasons I’ve occasionally heard, though I doubt they are all independently responsible for this curiosity: