There are two forms of universalism - (1) Christian Universalists who believe Jesus is the only way to God and that all people will eventually accept Christ at some point throughout eternity, (2) Universalists who believe that all roads lead to God or some form of ultimate reality. I cannot speak for (2) and while I do not belong to (1), I can understand the Christian Universalists position on sin.
For any Christian, there are not only negative reasons not to sin (judgment, confession), but also positive reasons. Positive reasons include:
Righteous living results in Shalom - if we love God and others with our actions, we make the world a better place for everyone. Sin is bad because it breaks us and others.
Obedience to God results in peace and joy that no circumstances can take away. For those with God's Spirit in their hearts, obedience to God brings joy because we obey out of love for God.
Obedience results in peace in our lives and in our hearts. It is the peace of knowing that we are at peace with God and others to the best of our ability.
True obedience to God is motivated by love for God; not chiefly by fear. So a Christian Universalist has every reason to obey God out of love.
Moreover, the Christian Universalists believes that one day all people will obey God - so why not start now?
In fact, George Macdonald, a well known Christian Universalist, used the following rationale to show that God's goal is not the punishment of sin but to bring every person to a place where they loathe and overcome the sin in their lives. This quote shows that to the Christian Universalist sin itself is the enemy to be defeated to heal humanity.
“Punishment, I repeat, is not the thing required of God, but the
absolute destruction of sin. What better is the world, what better is
the sinner, what better is God, what better is the truth, that the
sinner should suffer—continue suffering to all eternity? Would there
be less sin in the universe? Would there be any making-up for sin?
Would it show God justified in doing what he knew would bring sin into
the world, justified in making creatures who he knew would sin? What
setting-right would come of the sinner’s suffering? If justice demand
it, if suffering be the equivalent for sin, then the sinner must
suffer, then God is bound to exact his suffering, and not pardon; and
so the making of man was a tyrannical deed, a creative cruelty. But
grant that the sinner has deserved to suffer, no amount of suffering
is any atonement for his sin. To suffer to all eternity could not make
up for one unjust word. Does that mean, then, that for an unjust word
I deserve to suffer to all eternity? The unjust word is an eternally
evil thing; nothing but God in my heart can cleanse me from the evil
that uttered it; but does it follow that I saw the evil of what I did
so perfectly, that eternal punishment for it would be just? Sorrow and
confession and self-abasing love will make up for the evil word;
suffering will not. For evil in the abstract, nothing can be done. It
is eternally evil. But I may be saved from it by learning to loathe
it, to hate it, to shrink from it with an eternal avoidance. The only
vengeance worth having on sin is to make the sinner himself its