To me the word "tax" includes some expectation that the funds be used (however imperfectly) for the public good. But the bailout-and-austerity regimen seems to be about protecting financial institutions at common people's expense. For example, it wasn't the Greek people being bailed out, but rather the banks who lent to Greece. Yet we don't hear about that, we hear about irresponsible Greek bus drivers and the like. (As a side point, such fanning of nationalistic flames has served its diversionary purpose well in this case, but is really playing with fire.)
It seems to me the bottom line is clearly: are the bondholders of insolvent institutions expected to take a hit as well? If the answer is no, as it mostly has been throughout this crisis, that's "heads I win tails you lose" — rotten both morally and from a market point of view. The bondholders in this latest case are not touched at all, according to http://yanisvaroufakis.eu/2013/03/17/cyprus-stability-levy-a....
(You make a good point that this surprise tax isn't the worst way they could do it, but that's what makes it so puzzling politically. The way they're doing it is so naked, so brazen, that it's bound to cause a larger backlash. Everyone can understand someone reaching into their bank account and taking a fistful; that's political poison. Why didn't they find a worse way (if need be) to do it, cloak it in complexity, and exempt the poorest citizens to mitigate criticism? Something along those lines would be standard political practice, and it's puzzling why they didn't do it in this case. Is it simply that Cyprus doesn't matter? Edit: I now see that the Economist is asking the same question.)