The reason that this doesn't work for ebooks is that, unlike a physical book, you're only buying a license to use it (i.e. the same limitation you get on software).

I personally see this as a huge step backwards, and for that reason alone I'll be buying hard copy books for the foreseeable future. I'd love to have a Kindle or another eReader, but I just can't abide the limitations.

[1] http://en.wikipedia.org/wiki/First-sale_doctrine