like it's stated, it's voluntary.
now, on to the subjective. if i got an app from a white guy who didn't fill it out, i would ask him why not. if he gave any kind of answer like you just did, i wouldn't hire them because they seem like they have a chip on their shoulder about affirmative action.
any information obtained from you about your ethnicity cannot be used. that means they cannot use that information either to hire or not hire you. questions like that are used to map out a person's character, though. so, if you decide to not fill it out, have a different reason than you thought if you did fill it out it would be used to not hire you.
you'll also find that most questions on a pre-emp app are open questions, as in they cannot be answered either 'yes' or 'no'. it's all information gathering, and that information is used to find out if you'd be an asset or a trouble maker.
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