So Charles Dickens is the authority. Not the leadership of the South at the time. Praise God that this enlightened Englishman could correct millions of Confederate fools who were so badly misled by their ruling class!
Charles Dickens is an objective third party viewer who hated slavery and was a staunch abolitionist. Have you ever heard of this legal concept called "statement against interest"? They are generally regarded as true, because people usually won't make statements against their interests unless they feel compelled to be truthful.
The Confederate politicians would lie about their true motivation in the same manner modern politicians lie about theirs.
The unassailable fact was that the Southern states would have reaped a huge cash windfall from leaving, and the Northern states would face a serious financial crises as a consequences.
You tell me it's about some moral issue, but when people who have a lot of money at stake tell me that the thing which will make them the most money is a major moral issue, I think their motivation is money, not morality.
Especially after they passed the Corwin Amendment. You simply cannot jive that vote with any moral stance for attacking the Confederates. The Northern states were all so willing to keep slavery indefinitely so long as they could keep their thumb on the Southern state's economic production.
Money. It's always money.