I seem to remember there being some hullabaloo back in the day in the US over the issue of whether one person can own another. This is voluntary, but I don't be

I seem to remember there being some hullabaloo back in the day in the US over the issue of whether one person can own another. This is voluntary, but I don't believe you can voluntarily sell yourself into slavery, either. So I suppose the relevant difference is these shares grant the owner some percentage of all your future wealth, but they're never voting shares? They can't tell you what to do, so it's fine? Or is it just that no one has challenged whether this is legal or not?