Actually Im bit confuses too why the hell, he can still tweet or do some media interest over the public. Does prison usually allowed cellphon in jail? I think there is something wrong with the system of judicial here. Just because he is known personel that doesnt exclude him on a fair treatement like other criminals too.
I think he is in a kind of "soft" prison. I know that prisons and jails are divided into categories. For example people like bookkeepers will never be kept is same block with murder-assault-rapist-nazi-psycopath guy.
I've read an article about the prison (I dont remember the exact country, some northern Europe I guess), where a person is a prisoner, but can go to work to a near by office. Like he is from 9 to 6 at work, but after does not go home, but goes to his cell.
There is a "
Open prison" also. In Russia people for drunk driving spend half a year there.
Moreover, cellphones can be bought in prisons. Based on the google search, they cost from 200 to 1500 US dollars