Because they believe that their rules are the only way to learn to be 'good' and therefore discount all other paths. I understand this well as I am a secular humanist. I also ascribe to the rule 'and harm ye none, do what ye will' but it is not founded on religion, per se.

Based on scripture, all good comes from God. Entire book of Romans (Bible – New Testament) speaks of operations of the Holy Spirit through unbelievers. What I am trying to say is that our conscience is “God stamped” although, because we are given a will to make choices, we can override it. That could explain way back to kindergarten the “feeling” of good when we do something right and bad feeling/guilt when we do some harm, and all that without knowing all the rules of conduct yet. We are promised to have our laws written in our hearts and minds. (Bible – New-Testament, Hebrews 8:10) “…I will put my laws in their minds and write them on their hearts. I will be their God, and they will be my people.” I think that those that do good and live a sacrificial life by “instinct”, are still responding to God (creation can not ignore its creator.) I actually think that it’s with those people that God is most pleased. They are not doing anything to go to Heaven for, or out of the fear of going to Hell, but out of conscious and sincere desire to help another human (neighbor?). This burden or spiritual inert ion is called “Love” and it’s described in (Bible) 1st Corinthians 13. Finally, it helps to remember that in the Bible, Romans 2:13 is written: “For not the hearers of the law are just before God, but the doers of the law shall be justified.” I can just see God leaping of joy shouting “YES” it’s working… I am a Christian, but this ancient Muslim prayer speaks my heart desire. “If a want to know you just so I go to paradise, bar me from it. If I want to know you just so I would not end up in Hell, cast me there. If I want to know you because my heart desires to follow you, show me Thy will.” I would love to know that exact wording of this prayer. If someone knows it, it would be greatly appreciated if it get’s posted here.

As a Christian I have never had a difficult time understanding why anti-religious people 'do no harm.' To be honest I've never thought about it. People are either good or bad, this is not based on any religion but on the person. May be because I try not to judge is why I have never thought that people who do no harm means they have to be religious, and those who do nothing but harm must be without religion. May be I'm a freak and the only Christian who feels this way. I suspect that I am not the only Christian who see people and love them no matter what, whether they want to be loved or not, whether they do harm or don't do harm. Ah vell!! Kind of an offensive question towards 'religious' types, but sure it was not meant to be. Hope my answer helps your inquiry. In His grasp, <:))))<>< My salvation and my honor depend on God; He is my mighty rock, my refuge.--Psalm 62:7

Many but not all religious people believe that human decency is derived from religion or god solely, I however disagree because it is not derived from religion, it precedes it

Non religious people just choose to be independent thinkers free of religious dogma.Their moral values are no different than religious people."Do no harm" is more a common sense thing not associated with any particular religion.The morality of the public is dictated by customs and standards set in each country and has little to do with religion.

It doesn't fit with their idea of how things are. They think that god is the source of good behavior, so they are confused by good behavior not associated with religion or god.