Per default, the law states that the author retains all rights. They can license it, e.g. to their employer, exclusively; The employer can then sublicense that. However what licenses are possible is exhaustively defined in the law; on an abstract basis at least (e.g. using, creating derivative works, ...). It has been not exactly clear if conditions evoked on a license still have their roots in these licensable acts, or if they are based on contract law - where literally anything goes as long as it's not against the law or immoral.
Both can be enforced. However, I'd argue that it is good that it was decided that this is a copyright law matter, because this gives authors _much_ more protection than contract law, where all circumstances need to be evaluated for each single case and rulings might as well contradict each other.
Not a lawyer yet, though.