There is something to your point regardless, but moral disparities do not proportionately account for the disparities here. The situations are more complicated. The political types of topics which seem to suffer here are laden with some impurity with considering. Many cultural and moral values shift like the wind, so why do political concerns seem so immutable? I think we would be fools to accept this as simply a moral disparity. I encourage considering a more complex model of social relations. The more significant influences at play here are symbolism, identity and politics.
The right to not be a slave is a moral right. That's literally the definition of a right. First entry from Google's dictionary:
> morally good, justified, or acceptable.