Most workplaces block certain websites anyway so I don't see why they could not do the same thing for inmates. Maybe they shouldn't be using Tor, but I don't see why Wikipedia is a problem.
I believe prisoners need the ability to learn and improve themselves while in prison but the problem is much more complicated than ban tor and allow Wikipedia.
... though using torrents is generally recommended to reduce server load and bandwidth.
There's also nothing urgent about the access they get. Articles could be restricted to 1 or 2 years old to further prevent the transfer of timely information.
Its possible that lower security prisoners can be treated differently.
edit: typo
I'm only half-joking, I'm sure the prison authorities would be very concerned with this stuff.
Wikipedia certainly is not interested in the task of trying to track the IP addresses of each of these prisons.
(Also, I can guarantee you that the vast majority of these prisons are served by under-resourced barely-competent IT departments.)
(Also, it may not even be possible to distinguish between 'inmate' IP addresses, and IP addresses used by staff, or even staff working for the department of corrections not even physically at the prison. Why? Because there's no such thing as an 'inmate IP address', because right now inmates aren't allowed to use the internet at all. So you're asking under-resourced barely competent IT departments to set up a new IP address allocation regime. At 4500 different prisons.)
In Europe, and other places/countries with more homogeneous ethnic makeups, the incarceration doesn't seem to be that high. Scandinavia, for example.
Also, in fact, when you say "in other places with X, the incarceration rate doesn't seem to be as high", X can be _anything_. The US has the highest incarceration rate on the planet, by a fairly wide margin. So in EVERY other place, with any characteristic at all, the incarceration rate isn't as high. Not in Scandinavia, not in China, not in any part of Africa, not in Mexico, not anywhere. The US is alone.
Generally: the answer is to provide a web proxy or mirror with restrictions as to available content. And likely highly logged access.
I've done some peripheral work for corrections type people -- they don't adopt paranoid thought processes for nothing.
The average drug convict is not a criminal mastermind, but he is a cog in a criminal enterprise that is controlled by smart people. Drug and gang enterprises are real businesses, and they provide support and resources to their employees. They are capable of anything.