The article reports the effort as a "surprise tax".
Frankly I think taxing some part of revenue instead of just income on big companies would be beneficial, no more "investing" in making market harder to newcomers (or outright buying them out) and getting tax cut on that.
The point is that a windfall is also - by definition - a favourable change in the rules after the game has started.
The moral question here should be a simple one. Did the investors in the banks know (or at least reasonably expect) that they would make these profits when they decided to make their investment? If they didn't and they invested anyway then there's a reasonable argument for a windfall tax as long as it only claws back gains they had no reasonable expectation of making and did nothing useful to earn.