As he said, there is nothing to do, so to add some exitment into his daily life, and possibly gain himself an elite status in the jail with other inmates, he is able to waste government money on stupid law suites, so win or loose, everyone wins, and the tax payer pays.
Lawyers get extra work,
the inmates get status,
the inmates loose boredom,
A possibility of an improved menu,

Though 1 thought is, weight lose is reletive, as you get thinner, the amount of weight you loose at each point is less.
He may have lost more at the begining, such as 5 to even 10 pounds in the first week or or so while he gets used to the new diet, but then reduced weekly, till possible now loosing more like 1LB per week. on top of weight lose there is also water lose.

But true, this is completely stupid case waste of money. in the old days, hard labor, he would have lost a lot more.