Possessing the means of printing and distribution are sufficient, they claim. Yet by that reasoning, a publisher could in theory sell zero copies of a book for the life of the author plus fifty years, and still fulfill their contractual obligation. That is obviously not what authors want, what publishers really intend, or what the copyright law was written to address. Fortunately, some publishers have reluctantly conceded that some level of sales performance is necessary for their obligations to be fulfilled. Not much, mind you, but some.
Thresholds measured by dollar volume or unit sales are the basis for most existing compromises.
A major independent house has agreed to the same in principle, but at even lower levels of performance. In that instance, in the event that unit sales do not total fifty copies per year in each of any three consecutive years, the author can request a reversion of rights. Evidently the principle of minimum sales levels applies to bytes but not to bindings. At any rate, the giant does not speak with one voice. That may change, though, and possibly for the better.