Most people wont buy something for money that they could have for free. Not just for utilitarian reasons. Although on one hand the logical consideration is do I value whatever I'm getting in exchange more than the money I am paying on the other hand the perception that someone is getting the software for free, adding 1% value and then charging you creates the impression that they are getting an overly large advantage out of the interaction and nobody likes this.
People would rather be objectively worse off rather than feel that they had been taken advantage of.
If we look at it from a utilitarian perspective having someone who didn't add much capture all of the commercial value seems to be pretty poor. If the feature he was adding was worthwhile I bet he could have been honest and convinced people to pay him to add it. Everyone would have benefited and paying him would have been worthwhile and necessary because while he couldn't have legally kept it closed source, he didn't have to write or release it at all.
People would rather be objectively worse off rather than feel that they had been taken advantage of.
If we look at it from a utilitarian perspective having someone who didn't add much capture all of the commercial value seems to be pretty poor. If the feature he was adding was worthwhile I bet he could have been honest and convinced people to pay him to add it. Everyone would have benefited and paying him would have been worthwhile and necessary because while he couldn't have legally kept it closed source, he didn't have to write or release it at all.