CV boots are components of the front drive so if your front drive is covered which it should and there is nothing saying the boots are excluded then it should be covered too. Do you know which side CV boot is worn? When the CV boot cracks you get grease all over the wheel inside. That is one way to know but it does not make sense for them to wait for the boot to be cracked if they found out it was about to split. They may hope that it will crack after the mileage limit of your warranty is passed. I would fight on this one. Is this the BMW dealer? Do you have another dealer you can go to?