I'm confused by your statement. If "Ban != Access" then what's preventing courts from simply denying access (not a "Ban" under your assertion above) to people outside of prisons as well?
I think this would be relevant to someone who has committed a crime using a computer and remains banned from using the internet even after finishing a prison sentence (like the kid in the movie Hackers). It doesn't sound like that can happen now.
Suppose someone is caught with an internet phone in jail and is hauled in front of a judge. If it's banned, then they can have their jail term increased.