Why is it said that Sweden has a regressive income tax system? Quoting from Wikipedia


  Sweden has a progressive income tax, the rates for 2014 are as follows:  
  
  0% from 0 kr to 18,800 kr (~0 – 2,690 USD)
  Circa 31% (ca. 7% county and 24% municipality tax): from 18,800 kr to 433,900 kr (~2,690 – 62,140 USD)
  31% + 20%: from 433,900 kr to 615,700 kr (~62,140 – 88,180 USD)
  31% + 25%: above 615,700 kr (88,180 USD and up)[4]  


Why is it said that it is regressive?

E.g.

This post from Washington Post
Excellent question. You can understand this by reading the original study referred in the article. I will try to provide a short explanation here.

Basically the study argues that if you take into account all sorts of taxes like taxes on properties, consumption, dividends, etc. the European countries tax their rich residents relatively less than the poor residents, while the US does the opposite. This is possible because there are other significant taxes besides the income tax. As the study states:


  The main reason for this is the relatively low reliance of the
  American fiscal system on sales taxes, which constitute a much higher
  share of revenues in other countries (even when including state and
  local taxes in the US case, as the figures above do). General sales
  taxes place a disproportionate share of the tax burden on the lower
  end of the income distribution because expenditures are higher as a
  share of income as we move down the distribution.


To gain an exact understanding of the key claim in the WP article we have to understand both the Lorenz Curve and the Kakwani index. The Lorenz Curve has a pretty good Wikipedia page, I recommend browsing it. The Kakwani index is chiefly used to calculate payments for health services.


  The Kakwani index is defined as twice the area
  between a payments’ concentration curve and the Lorenz curve and is
  calculated as, $\pi_K = C - G$ , where $C$ is the health payments’
  concentration index and $G$ is the Gini coefficient of the ATP variable.
  The value of $\pi_K$ ranges from –2 to 1. A negative number indicates
  regressivity; $L_H(p)$ lies inside $L(p)$. A positive number
  indicates progressivity; $L_H(p)$ lies outside $L(p)$. In the case of
  proportionality, the concentration lies on top of the Lorenz curve and
  the index is zero.


Source

The difference in the Kakwani index is small though, the range of the graph in the article is in the hundredths.