The first sale doctrine exists specifically because with paper books, the content and the media cannot be separated. Although you don't own the content, you own the package (the paper) and can sell the paper should you wish, assuming you've bought it legally in the first place. The logic of this doctrine does not apply to eBooks which are separate from the media on which they're displayed.
Whether DRM is a good idea is a good question. So is whether readers should patronize publishers who impose it. As a small publisher who does not impose DRM on eBooks I publish, as well as a reader, I tend to agree that DRM imposes costs in excess of any benefits it offers. In my opinion, refusing to buy eBooks with DRM and letting the publisher (and distributor) know that's why you're not buying is a far more effective tactic than making flawed legal arguments.