Indefinite article after "as"

Hi, I read that when you classify or define people and things you use a/an:

Don't use your plate as an ashtray.

But then I found a news article which included the following sentence:

Indian teenager allegedly raped twice after cops failed to follow her after using her as 'bait'.

Shouldn't the correct sentence be "using her as a bait?"

My question is as follows:

1. Which is grammatically correct? Using her as bait or using her as a bait?

2. If the former is correct then why is the indefinite article not used after as? Isn't it defining her as a bait?

3. If both are grammatically correct then what is the difference in meaning or usage?

Any help would be greatly appreciated.

Thank you.