You need to reset it using VAGCOM. The airbag light will stay on at the mere hint of any malfunction to the system, no matter how trivial. It measures the voltage drop acros each component in the system, so if you were having low voltage issues before you took the battery out and turned the ignition on then this would give duff readings on the airbag system and would cause the light to come on. Unfortunately it's not clever enough to reset itself when the fault is rectified, thus the need for VAGCOM.