I'm not a theologian by any stretch of the imagination, but from what I understand sin is pretty much only a part of the three major religions in the world: Judaism, Christianity, and Islam (listed in order of appearance). And I think when you understand the general statements of these religions, you begin to understand why slavery and racism aren't considered sins.
All three have their roots in Abraham with the primitive, vengeful YHWH fellow. And from these roots we have "favored peoples" -- right from the get-go we see this whole supremist mentality being perpetrated. And as time passes throughout the holy tales of each religion, it just keeps on keepin' on.
In the Old Testament the Hebrews pretty much continue to engage in warfare and work about trying to establish themselves as a kingdom or political force in the region. Dominating others and maintaining the supremist postition is pretty easy. Things got a little more complicated with the New Testament and Jesus, but the last few bits in the life of Jesus make sure to hold onto the elitist mindset.
Although we see in the NT Jesus promoting equal rights or fair treatment with, say, stories like the Good Samaratin, he pretty much undoes it all before getting hammered. That whole bit about "the only way to the Father is through me" immediately restored the whole concept of supremacy. Only now it isn't necessarily about where you live or what culture you're from. Now it's about whether or not you accept Jesus
the Christ (it wasn't his last name, folks) as your lord and savior.
Just to address the issue of racism and slavery in America, the African-Americans were pretty much seen to have two left feet to white colonialists. Not only did they live in the "wrong" place, but clearly they hadn't accepted Jesus as their lord and savior. Since they couldn't talk or behave like the "civilized Christians" who were promised the Kingdom of Heaven for their rightousness, they were uncivilized humans whose only purpose could possibly be to serve those above them -- the arrogant white colonialists.
Ah, but I'm beginning to rant. The point is, the very foundation of the sinning religions is that of an elitist position. The concept of supremacy abounds throughout, thus leaving racism and slavery entirely justifiable to those wishing to justify it through religion. They interpret the Holy books to say whatever they wish to say... which isn't hard, when you think about it. They wouldn't be holy if they didn't have so many holes...