By referring to the terrorist group by its self-imposed title "Islamic State...", the United States government is legitimizing it as a governing body. Whereas, their Arabic name - Dawlat al-Islamiyah f'al-Iraq w Belaad al-Sham - is similar to another Arabic word - das - which means 'to trample down' or 'crush', which could therefore be the source of their dislike.
http://i100.independent.co.uk/article/why-isis-will-hate-it-if-we-start-...