Their thinking is that making the conditions bad will serve as deterrent i.e. would-be criminals would think twice before committing crimes because they're scared of going to prison.

Of course, this makes no sense, as most criminals have low impulse control and don't think about the consequences of their actions in terms of risk/reward calculations. We should use prison time to re-educate these people and try to make them better instead of psychologically torturing them, but here we are, and it's very unlikely things can change within the current political system (too many "checks and balances" for meaningful reforms)