A bike lane should be treated by motorists the same as any other lane. You don't cut across another lane to turn, do you? No, you merge into the lane closest to your turn before executing it. I would say a driver should merge into the bike lane, only after yielding right-of-way to all traffic in that lane, before turning.

From the cyclist's perspective, when approaching an intersection, it's helpful to move out of the bike lane and into the main flow of traffic. That way, its harder for drivers to pass you and hook you.