Slavery can only be imposed with the use of force. Since it is only the government which can use force to achieve its goals (in the US anyway), then only the government can institute slavery. Since a minimum wage (or any wage controls at all) can be viewed as fixing the value of someone's labor in spite of it's value in the market, then this could be viewed as a form of slavery.
Another spin on this might be that if the government forces one person's labor to be valued at a lower value simply to lend preference to another individual (or group) then this might be viewed as slavery as well.
So by not eliminating that minimum wage law, the government could be viewed as promoting a brand of slavery, but personally I think it's a harsh assessment.
I'm sure this is what you meant right? What does this have to do with the people who work at wal-mart of their own free will?
They work at Wal-Mart because there is no better opportunities out there.
Meanwhile Wal-Mart, and others fund the Cato Institute among others, to encourage massive immigration to keep the worker supply up, and wages down.