That's awful reasoning for using the cycle lane to turn, but (having never used it) I prefer the rule itself. Assuming the driver acknowledges they're changing lanes when they move into the bike lane, and checks appropriately, it's a clearer indication of driver intent and therefore precludes bike undertaking, whether deliberate or accidental. It also means the driver can focus on one thing at a time, bikes when moving into the bike lane, then the side street as they turn into it. I say this as someone who until recently commuted by bike almost exclusively and believes bikes should be given much more favour on the road. I'd be interested to know whether it's been found to be safer.