I don't get the thinking that exportation of former slaves is either desirable, doable, or a good thing.
Long story short, and crudely put: the leading proponents knew damn well that black people had grounds to be angry about how they'd been treated. One of them literally wrote that black slaves were like dogs that they had constantly beaten. To remove the chain would be to run the risk of the by-now really messed up and furious 'dog' to turn on its abusive 'master'. Conclusion: we have to shoot the dog, or send it far away.
Sending all the former slaves "back to Africa" was considered, by its advocates, the more charitable of the only two "realistic" options. First, keep in mind that most avid propenents of 'colonisation' were from the South. But then; even opponents of slavery generally believed that black people were inherently inferior to whites, intellectually speaking. This was believed so widely that Lincoln expressed such thoughts as late as the mid-1850s (
and was a proponent of sending all blacks to Africa). It appears that it was only the Civil War, and his resulting co-operation with black people of undisputable intellectual credentials - such as Frederick Douglass - that really changed his mind.
Of course, black people
didn't suddenly turn into rabid beasts as soon as slavery was abolished. Very surprising, how racist stereotypes can be so wrong, eh? Still, consider OTL modern day, with black Congressmen representing Southern states, and contrast that with the (certainly in the South) near-universal belief that blacks were inferior. The simple reason certain peoples supported exportation schemes was that they wanted to prevent a situation where back people could ever be living alongside them as their equals (or, God forbid, their
superiors). The fact that a whole lot of the exported people were going to die of tropical diseases hardly mattered to people who were fine using them as an expendable slave labour force in the first place.
So you see, once you start thinking like a completely immoral bastard, it suddenly makes a certain sort of sense.