I removed my ariel the other day, the thick black wire was cut at an angle on mine - wasn't attached to anything I think but I'm the 3rd owner of the car. I thought it something to do with if lightning hits the car, but just speculation.
Removing/refitting the earth is basically disconnecting the battery, no? Then you would need the key code. Ahh btw, when I changed my head unit from the original Alfa (clarion) unit to an aftermarket Nakamichi one, the ariel goes up and down 100% of the time. Anecdotal I know, but there ya go.