I wouldn't call it an eBook tax because taxes typically refer to compulsory fees that apply across an industry whereas any publisher is free to charge whatever they like for eBooks. It is more of a premium, but that is just semantics. The reason they can charge this premium is because publishers have a government granted monopoly (copyright) on the distribution of a given book and competing books are usually
imperfect substitutes.
The mystery is why, if publishers can make a higher profit on eBooks than pBooks, don't they always offer the digital alternative? My guess is that eBooks don't sell enough to make it worthwhile converting them and risking piracy. You could argue that eBooks would sell more if their price was much lower but I'm not entirely convinced this is the case. I'd wager that most people's book purchases are limited by the rate at which they can read books not how often they can afford them.
I think that publishers overestimate the amount of sales they would lose due to piracy and that in the long-term DRM or only selling books in a paper format will be ineffective as the eBook sharing community grows.