I think that the problem is that they are aware of modern lifestyle and in order to preserve confidentiality, secrecy (IMO it should be more open) and to avoid suggestions of external influence then tweeting, etc has been banned. I would think that the jamming devices are to stop eaves droppers otherwise the implication is that they cannot trust them not to use electronic devices.
Can I go back to the beginning? What makes you think it is illegal to jam a mobile phone signal? It might be illegal for a state to do it on a statewide basis but, surely, if an individual or company decides it doesn't want mobiles used on its premises, it can act to prevent that.
I am actually asking about this because I don't know what the position is. I just think there's no real difference between using shielding to block signals and saying, for example, no mobile phones at a swimming pool.
It is illegal to transmit any radio signal without a licence. there are a couple of wavebands that have a general public licence but cellphones bands are not one of them, the main ones are used for short range control eg car locks.
On the subject of jamming - GPS jammers are causing a problem with both aircraft and shipping. Drivers use them to block the signal so that their lorry owners cannot track them to know if they are swinging the lead or using the firms vehicle for private use. but they can cause interference the any other GPS signal in the near vicinity.