I hadn't considered this angle, and would love to be wrong, but haven't digital books been exempt from first sale doctrine for quite some time now? I thought this case law was well established.
I can't quote you law but my understanding is that first sale doctrine applies to physical things. So virtually lending a corresponding digital copy to a physical book seems a creative application of first sale doctrine--which may or may not be strictly legal but apparently the publishers weren't interested in pushing that.
