I think the point being made is that if you wave/flash to other road users, many think that you are indicating that it is safe to pull out, where as all you are saying is that you are letting them out.
If you see a motorbike coming in the other direction or in your right hand mirror would you still flash to let somebody out? Do you always check for other hazards before letting people out?
Its not your responsibility to do so, but what you have to remember is when many people see a flash, they completely forget all their normal checks when pulling out. Your arse might be covered by the law, but you might get somebody killed.
The worst culprits for taking waving/flashing too literally are pedestrians, especially children. You flash your lights at them and they cross the road without looking the other way. That is why it would fail you your driving test!