Your car battery is 12.6 volts, but when the vehicle is running it jumps to 14.4 because the alternator is charging the car and running the accessories. This is too much voltage for the motorcycle battery.
No offence but your wrong... right idea wrong reason.
why does everyone say not to run the car while jumping the bike....I just don't get it. Make sure you don't use your run switch to turn off the bike, even though it's the same circuit as the ignition key
The reason is simple, even though a cars electrical system is very similar to a bikes and the voltages are the same (13.2V no load battery and 14.4V charging) the current capacities are wildly different.
A bikes battery is rated at anywhere from 80CCA to 140CCA, a cars battery could be anywhere between 150CCA and 600CCA. Trucks can be anywhere up to 2000CCA.
This is what causes the issues, the starter motor pulls a certain amount of current from the battery and as a result there is a certain voltage drop across the wiring and the output voltage of the battery will drop a little as well. The starter motor is designed to work with this occurring and spin at the correct RPM. So when you add up all the losses in a system that is operating correctly you will find that the starter only gets ~9V - 10V while cranking.
When you throw a car battery in the mix and crank the bike over you have voltage losses across the jumper cables and in particular the clips where they attach, with the car off and floating at around 12V this is enough to bring the voltage down to a respectable level for the starter motor and no damage will occur.
If the car is running you have a 150+CCA battery and a 100Amp alternator operating at 14.4V the current potential in the system is so high that the load of a bikes starter motor is no where near enough to drag it down and even once you add in the losses from the jumper cables and what not you still get around 12V at the starter motor, which will make it turn over to fast and put to much current through it.
By placing to much current through the starter motor you risk damaging the windings and more likely the brushes.
Hope that helps Doof, and Mattieg now you see that its not the battery you should be worried about cause even at 14.4V it's still well within its normal operating parameters, its the starter motor.