Why can a non-clustered index not be dropped using online=on option
Because dropping of a NCI is already as much online as it gets. Is a metadata only operation. There is not even data deletion, a dropped index rowset is simply deallocated, ie. the same operation as truncate does. 

Dropping a clustered index, on the other hand, implies a rebuild and is a size-of-data operation, so it does make sense to have an online alternative.