I'm guessing that tuners hang a WB off the tailpipe when they don't have other options. They can't very easily weld a bung to the exhaust just to do a tune. Plus in many cases those cars probably are catless. The reason that a WB after the cat isn't as accurate is because the catalytic conversion creates and destroys O2 molecules (different than oxygen atoms), so there is more or less O2 molecules leaving the cat than entering it. The number of atoms is the same though. The O2 sensor only measures O2, so if those oxygen atoms are bound to other atoms, the sensor has no idea. Check out the wikipedia article on catalytic converters.
The reason there is a O2 sensor after the cat is to determine if the cat is working properly. It has nothing (or very little) to do with how the ECU adjusts fueling. That's what the O2 sensor in the manifold is for. The up-pipe cat probably does affect the WB reading, but since that cat is pretty small, the effect is probably pretty minor.