Some small fraction of authors' works are popular and produce lots of revenue. Copyright law in the US was explicitly intended to encourage these authors to get into the business. As the US Constitution says, "To promote the Progress of Science and useful Arts, by securing for limited Times to Authors and Inventors the exclusive Right to their respective Writings and Discoveries."
There are some authors who can and do make a living at their craft. What if they spend a year working on a book and then die just as it's released? The law can and should allow their rights be inherited, so that their heirs can benefit from the work, rather than having it immediately fall into the public domain.
The rationale why itunes song can be inherited, meanwhile, is even simpler. In general, all property can be inherited. Copyright doesn't change the fundamental nature of the property, it's just a restriction imposed on the property's owner -- even though the owner owns the property, the owner is not free to copy it. This does not change the property's capability of being inherited in any way.
So I see no contradiction. The copyright itself should be inheritable by the author's heirs, and copyright-protected materials should be inheritable by the purchaser's heirs.