If it were a one-off download, it probably wouldn't be. But I quite like the system O'Reilly uses: $5 for an ebook if you own the dead-tree version, because you can download it as many times as you like, in all different formats, and they maintain it with errata updates etc. for life.

The lifetime ability to download the most-current version of a book in any current format is, IMO, worth a few dollars more than a free but unmaintained ebook in what may very easily become a legacy format within a few years.

Good point. Every book should come with a free digital copy, but unfortunately the standard practice seems to be to offer a trivial discount when purchasing both.

The digital copy offers something the physical copy doesn't, e.g. portability. Some people might be willing to pay an incremental sum for this added benefit.