I've found the following. Vehicles in ArmA are considered by AI friendly or enemy just based on vehicle type.
Just an example, I spawned a Camel piloted by an OPFOR pilot flying towards my group (all armed with Stinger), none reacted to it until the camel was over our heads. Then the test was repeated but using a CamelE, and it was toasted immediately. So AI consider a vehicle west or east based on type, unless any AI is close enough to determine real side (crew) or empty. IMO this is not a bug, but a realism factor.
In your case, may be the AI was not fully aware it was an empty vehicle. You may try getting close to it with your group, so everyone knows it is empty, and then destroy it.