It’s a deliberate misrepresentation of the data. Yes they have the highest number of groups and you’d think that would be logical in that they have the largest population. But per capita, SPLC ranks Montana number 1.
http://247wallst.com/special-report/2017/02/17/10-states-with-the-most-hate-groups-2/4/
Don’t know about the overall newsworthiness of the site, but the info they use was obtained through an interview with the SPLC, and seemed properly sourced.