Otherwise the NYT would have told readers that the legislature had supported an infinite percentage increase in the state income tax. Telling readers that Illinois increased its income tax by 66 percent provides little information to most readers, since they are not likely to know Illinois's current tax rate. (This does appear lower down in the article for those who read far enough.) Furthermore, "66 percent" invites confusion with 66 percentage points, which would be a devastating tax increase.
As it stands, the increase in the tax rate was 2 percentage points, from 3 percent to 5 percent. It would have been more informative to readers if this information was provided at the top of the article.