I would think he's in protective custody because he would be in extreme danger in general population. He's a celebrity and would likely be viewed as rapist and traitor (the facts don't matter, what matters is what the other criminals would think).

A good question is whether he needs to be held in a maximum security facility. He's a bail flight risk, but is he an escape risk? I would think he's at most a moderate escape risk (has some capabilities and has shown his contempt for the legal system previously) and would be fine in something less than maximum security (so long as protective custody could be maintained).

I do know Assange has a competent legal team that would fight his being held in maximum security if Assange wanted them too. I wonder why he hasn't instructed them to do so.