Strange 4:3 pillarboxing

That's a different matter - the original production company or distributors decided to go back to the original film stock and remaster it for HD, which happened to have extra unused space to make it 16:9 with minimal cropping on the top and bottom of the frame. Nothing to do with Comedy Central, they just happened to pick up the rights when the HD remasters were beginning to be shown worldwide.

Not that you could do that with the Fresh Prince anyway, as it was shot on video.

5 USA are doing this on their new Remington Steele repeats, so it looks like going forward 'new old' shows on Channel 5 in 4:3 will get the blurry pillarboxing treatment

Meanwhile, Fresh Prince is also being shown on MTV at the moment and they're happily showing it with black bars.