The reporting on this is a bit hazy right now, according to CNN it was not a formal briefing but a "press gaggle" (https://en.wikipedia.org/wiki/Press_gaggle ht

The reporting on this is a bit hazy right now, according to CNN it was not a formal briefing but a "press gaggle" (https://en.wikipedia.org/wiki/Press_gaggle https://en.wikipedia.org/wiki/Press_gaggle) and the CNN reporter was barred by a WH staffer because the organization's name "was not on the list" (http://www.cnn.com/videos/politics/2017/02/24/cnn-blocked-from-white-house-press-conference-murray-nr.cnn http://www.cnn.com/videos/politics/2017/02/24/cnn-blocked-fr...). This is a mostly symbolic gesture (these organizations still have their assigned seats in the room) but is very much against WH press tradition. Coupled with Trump's strong words at the Convention this is a sign for these organizations to tune down their criticism of the President.