The reason they tell you not to bump start a car with a catalytic converter is because if unburned fuel gets to the catalytic converter and burns off inside it can damage it very badly.
The truth is though, out of all the times I've had unburned fuel get into the exhaust the only time I've ever had a catalytic converter burn up was on a car with a realy messed up ignition system. This car had completely destroyed contacts in the distributor cap. It missed so badly it sounded like a WW2 fighter plane after the engine was hit.
If you burn the catalytic converter in a car you will know it, it is the most discusting smell you will ever get from a car. It also sticks in your nose, All I could smell for the next week was burning converter.