Whilst I don't disagree with the point or pointlessness of jamming mobile phone signals, if you believe a simple thing like the law will stand in the way, you're sadly mistaken.
The police and government (including various state bodies) break the law all the time and generally get away with it. Always have done and always will. The argument that jamming mobile phone signals is against the law is not sufficient reason to stop them. However, to be fair, most of the time, they just tell the mobile phone companies to turn the transmitters off rather than mess around tryign to jam them!!