There's always the "good" reason and the real reason. The "good" reason they're using to justify cell phone jammers is to stop criminal activity. The real reason they want to use cell phone jammers is money. Prisoners with cell phones don't pay the exorbitant fees to use the prison phone or send an email. I'm sure some of these cellphones are used for criminal activity but I suspect if the cost of staying in contact with family was reasonable they would see a significant decline in smuggled cell phones without needing to jam any signals.