Publishers may find used ebook sales desirable if it is accompanied by an increase in ebook sales: as a means of capturing consumers who wouldn't purchase ebooks or as a means of translating piracy into sales. Textbooks were provided as an example, primarily because a lot of textbooks are resold. When I do hear of people using electronic versions, it usually involves some sort of piracy. Now some of those people are compulsive pirates and who wouldn't buy a book even if they could resell it, but some of those people simply don't see the value in spending upwards of a hundred dollars on a book that they will use for one semester and cannot resell. Worse yet, some of those electronic textbooks expired after a certain duration.

It is also worth considering that legally reselling ebooks that contain DRM, in many markets, will require some sort of mediator in the transaction. It wouldn't surprise me if the publisher received royalties from this reseller, if the resale royalties didn't go towards authors (at least until contracts are renegotiated), and the publishers maintained some sort of control on pricing in the process. All of that would benefit the publisher.

On top of all of that, publishers may be forced to accept resale in the long run. Some countries have strong consumer protection legislation and will increasingly see the prohibition on resale as an attack on consumer rights. That is especially true as more and more physical goods are being replaced by digital counterparts.