The Honda doesn't need a fully charged battery, just a battery it can charge while it runs. Leaving the cables on the car was reversing the charge back into the bikes system because it is a higher amperage. You can boost a bike with a dead battery and if it WILL take a charge the bike will happily keep running and do that.
I bought an '82 KZ1100D new and had no intention of ever selling it, but that's another screwing I took and don't like talking about.