Rights always supersede laws. After all laws can give governments the right to trample all over peoples God given rights. ie. slavery, the holocaust, communism's’ collectivism that starved to death millions in both the Soviet Union and China as well as many other places. Laws are only as just as the people who pass them.
On the other hand, as was pointed out elsewhere on this thread, this law doesn't prohibit people from speaking freely without a license, only for charging a fee for their speech (guidance)without a license. If they want to give free tours I believe they may.
Was slavery actually codified as acceptable law or was it simply a practice that was so common at the time that no one thought ill of it?
I find it hard to believe that slavery was permissible by law and statute, but to be honest, I wouldn’t be surprised if it was.