The NCR Bazaar at the capital had slavers plying their trade openly because of a simple loophole. If the NCR was truly, vehemently opposed to slavery, Congress could have ironed out that little detail in days.
Its not the NCR Bazaar its the bazaar outside the NCR Capital. And a legal loop hole isn't a condonment. It should also be noted that during Fallout 2 the northern border of the NCR more or less ended at the Capital new territory to the north wasn't acuired until after the games conclusion.
Here's an example of a legal loop hole not being a condonement.
In 1993 Prince(the musican) wanted out of his contractual obligation with Warner Brothers. He couldn't get out or publish an album with another company while using his name. By changing his name to an unpronouncible symbol he exploited a loop hole on the contract. The day after the contract expired he went back to using Prince. By your logic Warner/Chappell condoned Prince screwing them over.
Most often a loop hole is an accident.
If the NCR condoned slavery then why do the NCR rangers reward you for killing them? Why are fighting slavery across the north? Just because some criminals find a legal loop hole to avoid punishment doesn't mean there actions are in anyway condoned. We don't now the details of the loop hole or how long its been there. The NCR couldn't act directly because if the government isn't bound by the law then no one is. Thus they look the other way if you solve the problem while the law is backed up in congress. A law can't be changed in days, with how legal language is how exact things need to be, they need to close the loop hole with out interfering with legitimate business. Its not that easy.