To paraphrase @popehat, this is just how federal prisons are. (OK, I don't know how such prisons are in the UK, but presumably they're bad.)

That's a Bad Thing, for sure. But I suspect most of Assange's fans aren't upset about the treatment of Assange because they oppose the carceral state; it's because they're fans of Assange.

In other words, if you say "I'm opposed to the conditions of Assange's detention", I suspect you're not saying you oppose the normal conditions of legal detention--which perhaps you should--but, rather, that you are opposed to the fact that those norms we happily apply to millions are being applied to Assange.