Prisoners, whilst not enjoying liberty to go out and about, make demands on the system for resources. They have committed a crime against society, and it is in the public interest that they are locked up. With human rights in place, prisoners have successfully gain TVs, video games, and a food/drink menu that is better than most of the population enjoy (and per inmate is higher than the investment in school meals per child).
Not only will the removal of luxury items be a cost saving for the prison service, the removal of human rights will allow a greater remit for initial crime punishment, but also as a 'carrot-stick' system for ongoing behaviour whilst in jail.
Final point – soldiers are not allowed human rights on the battlefield (recent ruling) so why should prisoners?