Hi,
I had the exact same issue on my IS.
Basically you are correct when you said Lexus have a policy not to supply bushes but the whole part instead. The reason is because it's a lot easier for a mechanic to replace the whole part. To replace bushes you have to remove the control arm/wishbone then remove the bush then replace the bush then bolt the part back to the car. By cutting out the middle process of sorting the bushes, Lexus make more money on parts as bushes are not expensive but wishbones are. It also means their mechanics aren't tied up on a job thats taking 2 hours when it could be done in 30 mins.