The pretext is that this will allow the police to track phones reported as stolen, when coupled with the time stamp of security cameras. The system automatically alerts the authorities when a lost or stolen SIM number is located nearby...
Source: http://www.phonearena.com/news/Creepy-Moscow-outfitting-metro-stations-with-SIM-card-snooping-machines_id45821
Source: http://www.phonearena.com/news/Creepy-Moscow-outfitting-metro-stations-with-SIM-card-snooping-machines_id45821