I explained why... if you didn't understand it, I can explain it again a little differently.
With the negative (or positive for that matter) terminal disconnected, the charger, maintainer, tender or whatever ONLY has a circuit to and with the battery. That means that the 'smart' circuitry in the maintainer which monitors the battery charge isn't being fooled by incidental draw from the car's ancillaries such as the radio station-set memory, or any other appliances - clocks, brake or dome lights, or 4-way flashers for instance, which continue to draw current when the ignition switch is OFF, not to mention any possible shorts.
On a Bike, none of it's appliances work unless the key is in the ignition and switched to ON or ACC, not the headlamp, turn signals or brake lamps, so there is no need to disconnect the battery from the Bikes circuitry.
On a car however, there are lots of appliances which do not require the ignition to be ON or at ACC to draw current. If you do not disconnect the battery from the car's circuitry, there can still be a current draw which the maintainer can mistakenly interpret to be a battery not at full charge and so it continues to send a charging current when it shouldn't.
Hope this better explains it.
Cheers!