But, as an ancient law, possibly left as a means of cultural preservation, it is still very problematic as it allows modern day (future day?) slavery.
T'pring also seemed like she would be in control because she says, "if you leave I would have your name and your property* and Stonn would still be there" It doesn't sound like she is the property in that assessment.
*Does Spock have a house in the 'burbs? An apartment? Maybe a foot locker full of old trophies? It makes me what property Spock has.