I think you are ethically correct to do so. I even think you would be ethically correct to lend out your only copy (as long as you really were certain that it was and would remain the only copy, so, lending it out to somebody who was completely trustworthy).

The only thing I question about all this ethics-wise: is it really ethical to purchase the item in the first place? I feel bad about all my Amazon books at this point, I’ve spent a bunch of money supporting an ecosystem that harms people who aren’t technically savvy enough to break DRM (actually, technically this includes me, I’m sure I could, but I haven’t looked into it or put the software together!). I do worry that solutions like this make the whole bad situation more bearable for the most-engaged segment, who could otherwise be leading the charge to something better.