There are two things here that caused the collision
@Paris 182 managed to avoid;
1. Wholly inappropriate courtesy
2. Taking a flash of headlights from someone you do not know as confirmation that a situation is safe
Why did the BMW stop to let the biker out?
To feel good that he'd helped one person out for the day? What about the queue of cars behind him that had to stand on their brakes as stopping in a line of moving traffic is the last thing they expected him to do. Help one person out while pissing off several others in the process by doing something unpredictable = retarded.
Why did the biker feel that a flash of headlights from a driver who's a complete stranger to him was all he needed to then move away with his eyes closed? Because he's an idiot.
I let people out if I can do so without inconveniencing others but I never flash them to indicate it's safe to go, I leave space and let them decide. If they sit there unable to make a decision I move on again. If you flash you're leaving yourself open to a world of pain when someone dies as a result of your actions. If you go when someone flashes you without being certain you're safe to do so...you deserve to have an accident!