By doing that the CSA politicians deftly danced around the issue of property "rights" to one of simple property.
In practice then it was up to the property owners to make the choice, hold their property and earn the nation independence, or hold their property and loose. In effect, the CSA measure that finaly passed was not an emancipation bill, it was sort of a "fundraiser", seeking donations of slaves rather than cash.
It would be as if, to pay for the wars in Iran and Afghanistan, the USA government, of Bush, standing on a principle of low taxes and low debt, instead had funded the wars by asking its conservative base to send in free-will donations of money.
So the question then became, how far and how willing were CSA property owners to go in free-will donations of their property to ensure independece? It was left up to them to decide, your property or your freedom. What did the large majority of slaveholders decide was more important even at this late date, keeping their slaves or securing their independence?
But then that questions isn't quite accurate either, because the CSA measure didn't allow a state to donate more than 20% of their slaves. A state could free-will donate all of their white sons, but only a fifth of their black slaves. Imagine if US governments since that time fought all our wars that way: we will draft all of your eligible sons, but you can donate no more than 20% of your cash.
Is it that white sons were not as useful or productive workers as black slaves if they were kept at home, or is it the CSA politicians just could not let go of the institution, even if it was by free-will donations? In any case it is a moot point. They never anywhere came close to a 20% mark. Property holders across the CSA were now given a choice, by thie legislation that finally passed, freely donate up to 20% of your slaves, or give up any notion of obtaining independence.
Everywhere the reponse was underwhelming. They chose to keep their property. It was a free choice made by these people themselves, no government forced a choice. The government drafted their white sons by force, requistioned horses and mules and other commodities, but would not force any proprty owners to surrender a slave. Nowhere did property owners rush forward, in anywhere near the numbers needed, to help make their own independence a reality.
Granted many white sons volunteered to go, and many black slaves served also in whatever capacity, but when enough white sons did not volunteer they were drafted. When not enough black slaves "volunteered" the government would not draft them, but would merely encourage and accept donation, up to a 20% cut off, and could not even get that much.
Did the politicians, the military and the property owners of the CSA give up the last and only remaining hope of winning their own independence, in order to hold onto their property when offered a free choice, even as late as 1865?
Without a forced draft of white sons, how long would the CSA have lasted? Realizing the odds the CSA government drafted them to keep their hopes alive. (And yes they did so in the North as well.) But, how long might they have lasted, or maybe even won their independence, if an equal measure had been applied to the other black half of the southern population?
As Pam said, property rights seem to have been a higher priority. Higher than independence, much higher than even the personal liberty rights of white draft age men.