Because it leads to a utilitarian attitude to people. Though this is kind of speculative.

What's wrong with a utilitarian attitude to people? It certainly doesn't feel "cold and inhuman" to me to point out that people prefer interactions with others that they stand to benefit from.

It should also be pointed out that utilitarianism and benevolence are hardly opposed - people naturally gain satisfaction (i.e. utility) from the happiness of other human beings. Our natural empathy means we don't like seeing each other in pain, generally speaking.

Emotionalism is an odd phrase. You mean you disapprove of that emotion. Emotions are the key to motivations. A person cannot live by logic alone.

I think we're going to strongly disagree on meta-anthropology. But by "emotionalism" I don't mean "emotions" per se; I mean treating emotions as if they were roads to knowledge.

Commerce is of course not evil but in treating friends like commodities will sound cold and possibly evil.

I was talking about treating friends as transaction partners, not commodities. Commodities are the objects of transaction. The fact that you instantly jumped to this extreme interpretation of my words is a little troubling.

Mercenary is a slur. Machiavelli always preferred loyal soldiers who believed in the state rather the swords for hire. Loyalists fight to the death.

Machiavelli is the walking embodiment of treating people like "commodities" or as "means to an end." His preference for loyal rather than mercenary soldiers was exclusively a practical preference based upon enhancing the power of the State. He thought loyalty was good because it was useful... surely that is worse than what I'm talking about.

If everyone is a friend for hire what is a friend?

Seeing people transactionally doesn't mean everyone is "for hire" in the sense of money. The idea of a friend is that the pleasure the two people get from each other's company is a non-monetary payment in and of itself.

Psychopaths see people as humans they just don't have an empathy. They have little fear, guilt or sense morality. They no what is wrong but they do not feel it.

What is it with this moral sentimentalism? Are you suggesting the only valid morality is based on feelings?

They only respond to rewards.

People respond to incentives. What is wrong with this?