Allowed? Every single state was a slave state in 1776. I think the first state to stop slavery was Massachusetts, and they only did it through a ridiculous legal trick by an activist court. They didn't vote to get rid of it.
I'm not so sure that slavery was a big issue for the states in 1776. Later it became more significant, but I don't think it was all that big in 1776.
If the founding fathers had insisted, there was a really good chance the country was never founded.
The Committee of five stripped out all of Thomas Jefferson's anti-slavery rhetoric from the original draft of the Declaration. With all 13 states making money from slavery, it was very foolish to even put that in there. John Adams and the rest knew better than to indulge Jefferson in this topic.
There would never have been a USA if they had started out on that foot.
So riddle me this, why would these life insurance companies risk insuring slaves if they were abused or treated like machines and worked to death? Answer - because they were not.