I don't think you need to say it. The word "slavery" already carry the implication that it was about the slaves, the blacks. When non-historian (and even some historians) people talk about slavery in the American context (and it's often misunderstood as such in other contexts), they think about the horrible treatment of black people; they don't think about the institution as a whole and why it existed. (This is probably a good thing, but it calls for careful wording when dealing with issues like the civil war).
Admitedly dishonesty was a harsh term. I'm sure most people are just stating a honest opinion; the problem is accidental, not deliberate.
As for racism being a basis of the institution of slavery, that's certainly true. But, while slavery was a racist institution, that's not all it was (that is, all slavery in the US *was* racist, but it was other things at the same time too), and as it happens, the south seceded and fought because of other aspects of slavery (the "support our own place in society" aspect, largely), not the racist aspect.