They are prohibited from seeking the information by any means (including background checks), except information which is legally public, such as when the prospective employee is a previous/current public employee.

some jobs background checks make sense. do you really want to hire a former bank robber as a teller? or convicted sex criminal as a teacher.

Personally, I would want to do a credit check on a CFO or controller because if they can't manage their own finances how can they manage the businesses.

Employers generally can't get your salary history during a background check without your approval.