> It's the same reason the IRS gives you a spot to declare your bribes and other illegal income.

The California example makes sense. They aren't asking a question that would lead to the admission of a crime. The IRS example doesn't make sense, since they are asking a question that would lead to the admission of a crime. Even if the answer was legally protected, a government who does not respect the law (or one that changes the law) could have nasty repercussions.