Can an employer rescind a job offer if your background report shows a clean criminal history, but I self-reported an offense from over 10 yrs ago that did not lead to a conviction?

I received a job offer contingent upon passing a background check. I did pass, clean criminal history record. BUT I self-reported an offense from over 10 years ago that happened in Iowa that resulted in a fine of $250. I did not know if it would show up on the check, even though I do know CA law can only go back 7 years, but the question the 3rd party agency asked was "have you EVER been convicted..."

Now the employer is considering taking adverse action on my clean background check. Since I self-reported an offense that could not be verified, the employer is considering rescinding the offer. Is that legal?