The drawing is a rendering of a SuperMax cell. It serves the purpose of keeping him in rather than keeping him safe although it certainly does that as well.
Much as we hate the criminals in these cells, in some cases it is unsafe for them to be allowed human company. Legally and morally, people in custody must be protected, even from suicide, no matter how we (outsiders) feel about it.
Lets say by some horrible misapplication of justice you were to spend the rest of your life in there or be "put to sleep". Which would you choose?