- "It is believed that the Russian move is a tit-for-tat exercise in response to sanctions by western countries following recent events in Ukraine. However, the Russians say that the action was initiated by the refusal of the U.S. to enter into negotiations on the placement of Russian-operated GLONASS tracking stations on U.S. territory. "
http://gpsworld.com/russia-turns-off-data-from-igs-gps-tracking-stations/
Our refusal to allow GLONASS tracking stations in US territory has been a bone of contention for quite a while... predates the Ukraine situation considerably, although that might be what finally made them pull the plug.
One of the problems with using GLONASS that was always cited by the manufacturers was that their orbits were inaccurate, due in part to the fact that they couldn't track their satellites 24/7.
Edit: Apparently that was then and this is now; I stand corrected:
What isn’t widely known is that Roscosmos already uses sites on U.S. territory for monitoring the availability and health of the GLONASS satellites as the map below clearly shows.