Because wikipedia is editable by _anyone_ it's probably not the best for prisons - it would provide a means for communication that wasn't able to be monitored.
I mean, if you were in prison and had access to Wikipedia, I could edit, or put something on the talk page, that was a message to you.
You would look up the specific page, and get the message.
There are downloadable, offline versions of wikipedia.
This is an excellent point. Wikipedia is explicitly not a primary source so any access to it should include the sources as well.