I think it is appropriate that the Huhnes are transferred to open prisons, but the key in the phrase is not the word 'open' it is the word 'prison'.
Although the regime is far more relaxed, it is still the curtailment of liberty, and being locked up with a load of strangers.
Never underestimate the power of personal freedom removal, it is something we all take for granted sitting at home in front of our PC's - I really don't think it is quite the 'cushy number' a lot of people imagine.