The aim of the sanction was to penalise the regime and put pressure on it to stop its war against the rebellion. By lifting sanctions on those who defect, the EU is creating an incentive - if a soft one - to defect from the regime and weaken Gaddafi's position. It is unlikely that there will be many defections now as the stalemate in Libya means that the regime isn't likely to be ousted, so those who decided to stay probably won't jump ship now. Still, the move may pay off in the future if the situation on the ground changes. Then members of the regime may see a life boat, and take the plunge.
It can be a tough decision to ease the position of people who have participated in such regimes, but the hope is that by removing them peacefully with incentives, bloodshed would be avoided or reduced than by backing regimes that are clearly capable of great cruelty into a corner with nothing to loose.
From EU Observer the routine attitude with which the sanctions were lifted seems to indicate that this is an accepted strategy across the EU:
"The EU decision was made unanimously at ambassador-level last Thursday.
Foreign ministers rubber stamped the move without debate or public remarks at a meeting in Luxembourg on Tuesday. Koussa became legally able to once again travel inside the Union and take money from any bank accounts he has in Europe from Thursday morning onward."
Jumping ship is still a risky move for members of dictatorships: though they leave with promises of immunity or certain privileges, their legal situation has a habit of changing eventually.