Non-slaveholders fought for slavery because secessionists persuaded them that they benefited as much or more from slavery as slaveholders did. Here's James McPherson in
Battle Cry of Freedom:
Nevertheless, the partial correlation of cooperationism with low slaveholding caused concern among secessionists.
So they undertook a campaign to convince non-slaveholders that they too had a stake in disunion. The stake was ****. In this view, the Black Republican program of abolition was the first step toward racial equality and amalgamation. Georgia's Governor Brown carried this message to his native uplands of north Georgia whose voters idolized him.
Slavery "is the poor man's best Government," said Brown. "Among us the poor white laborer... does not belong to the menial class. The negro is in no sense his equal... He belongs to the only aristocracy, the race of white men." Thus yeoman farmers "will never consent to submit to abolition rule," for they "know that in the even of the abolition of slavery, they would be greater sufferers than the rich, who would be able to protect themselves... When it becomes necessary to defend out rights against so foul a domination, I would call upon the mountain boys as well as the people of the lowlands, and they would come down like an avalanche and swarm around the flag of Georgia.
Much secessionist rhetoric played variations on this theme. The election of Lincoln, declared an Alabama newspaper, "shows that the North [intends] to free the negroes and force amalgamation between them and the children of the poor men of the South." "Do you love your mother, your wife, your sister, your daughter?" a Georgia secessionist asked non-slaveholders. If Georgia remained in a Union "ruled by Lincoln and his crew... in ten years or less our children will be the slaves of negroes. "If you are tame enough to submit," declaimed South Carolina Baptist clergyman James Furman, "Abolition preachers will be at hand to consummate the marriage of your daughters to black husbands." No! No! came an answering shout from Alabama. "Submit to have our wives and daughters choose between death and gratifying the hellish lust of the negro!!... Better ten thousand deaths than submission to Black Republicanism.
To defend their wives and daughters, presumably, yeoman whites therefore joined planters in "rallying to the standard of Liberty and Equality for white men" against "our Abolition enemies who are pledged to prostrate the white freemen of the South down to equality with negroes." Most southern whites could agree that "democratic liberty exists solely because we have black slaves" whose presence "promotes equality among the free." Hence "freedom is not possible without slavery."