Why does asking about salary suddenly make the interview awkward?

I brought up salary during the interview because there are bills that need to be paid, and food isn't free. The recruiter went quiet for a moment, as if I'd asked him to give me access to the company's bank account.

I'm not trying to be rude or greedy. I just need to know whether the job will cover the rent and leave me with enough for dinner. Why do some employers treat the simplest question like a personal attack?