Safety
These jammers are dangerous... They are dangerous for the people with electronic implants (like pacemakers), because a jammer could disturbe a feedback loop. They are dangerous for trains, because the new safety communication and signalling standard uses gsm transceivers (the standard is called GSM-R), And they are dangerous for the user, because if someone happens to jam the conversation of a police officier the chance is high that said officier would only use shoot to stop rules against the user (aka. 'terrorist').