Is it that they were deemed able to own themselves, and so were protected under property law? I'm just trying to understand the social and legal constructs that allowed for some blacks to be free (though second-class citizens) during that time.
There are numerous examples of this actually happening to black people. Not just in the South, but multiple free black men in the North were kidnapped and sold into slavery down South. If you didn't have documentation on you that said you were free, simply walking down the wrong road could mean the end of you being free. And again, even times when free men had the necessary documents, they were ignored and sold into slavery anyway.