Are they? Apparently, Federation traders tacitly approved of the slavery in "Cloud Minders" until Kirk decided to visit and got into trouble. Our TNG era heroes haven't faced the issue of slavery too often, and certainly haven't taken active steps to end such practice where they meet it.
And it's not as if the Feds even know all that much about the Klingons as TNG opens. In the second season, they are still fearsome and alien; in the fifth season, we learn they used to raid UFP planets and installations until just a few years before TNG, while formally in alliance. The Feds at that point are still far, far away from being able to tell their allies how to behave.
Timo Saloniemi