Because you seem to believe that people are basically good and when they do sin its because of some outside influence...you don't seem to believe that people can be evil all on their own,that they can be the source of their own evil and therfore fully responsable.
God created us in His image and declared all of His creation good - Genesis 1:26-31
He also gave us dominion over all of creation - including the Tree of the Knowledge of Good and Evil and the Tree of Life and even the serpent. Unfortunately, Adam and Eve submitted to the very things they had dominion over......the very act of listening to the serpent was a misuse of creation and an act of idolatry. Therefore, they lost their birthright.
We learned to sin and blame creation for doing so (trusting our own judgment over God's) from the serpent - Genesis 3:13
There is no mention of absolute depravity in the story of Adam and Eve - they were cut off from the Tree of Life and kicked out of the Garden - life became harder for them for sure, but they were still created in God's image.
Therefore, they remained basically good, but had the tendency to sin - and because of their broken relationship with God, sin (self reliance) became their first choice in all circumstances.
Today, Christians have a relationship with God, once more. We no longer have to make selfish/sinful choices.
Unfortunately, not everyone is a Christian, we share the world with people whom God loves, but choose to live outside His laws.
So this is where the heart of our disagreement lies - are Christians responsible for the choices/behaviors/lifestyles of nonbelievers? The answer is no. God allows the sinful to condemn themselves.
Nonbelievers have the freedom to live as secular humanists or hedonists or whatever they choose to do. In America, they have the freedom to live this way without persecution of the government if they are adults and do not present a danger to society or individuals.
What I do not understand is why certain Christians believe that our country must enforce God's laws on nonbelievers - as if the bad behavior of nonbelievers is going to invite God's wrath on the rest of us. God gives us freewill - if nonbelievers want to squander it, God permits them to do so - why are we trying to usurp God's authority over nonbelievers?
If I were a secular humanist, as you claim - I would believe that all humans were capable of immeasurable ability without God -that we could be both moral and immoral without contradiction or consequence - that we were our own god and responsible only to ourselves. I do not believe this idea at all. Without God we are lost. Ayn Rand is the epitome of secular humanism, which is why her philosophical ideal is embraced by Satanists - not sure why so many Republicans who claim to be Christians like her writings as well. In my opinion, her social Darwinian understanding of human existence is the antithesis of Christianity.
Finally, Christians are not called to babysit the damned. They are just as free to make choices as we are.
As far as believing outside influences make us sin - that cannot be farther from what I believe. If I believed that I could not be a counselor. If I believed people were not responsible for their actions I would not be able to help anyone make positive changes in their lives - I would just be sitting there in a counseling session agreeing with the latest victim of life sitting across from me.
Conversely, I see Christians blaming the Devil or demons for their bad behavior, all the time. Also, if you really believe people are completely depraved, how can you hold them responsible for anything? If people were completely evil - all their sinful behavior would be excused (mentally challenged people are not responsible for their inability to make reasoned, adult decisions, for example) and the blame would fall on God for not fixing them.
I believe people are basically good, but broken; God's sanctification is the fix. It is our job to practice our sanctification by loving others, sort of like an injured athlete has to participate in his own rehabilitation if he expects to regain his former abilities. We can only change the behavior we take responsibility for - and we can only change it within the context of a relationship with God.
So as a counselor, I help people take responsibility for their strengths and their weaknesses - if they are Christian, I remind them that as a child of God, they are created good and no longer have to live in the bondage of sin. If I believed outside forces controlled their behavior, I would have to believe that they were victims and teach them how to hide from life.