Here is the rub. They are required by law to tell the potential employee why:
"You must be told if information in your file has been used against you. (emphasis mine) Anyone who uses a credit report or another type of consumer report to deny your application for credit, insurance, or employment – or to take another adverse action against you – must tell you, and must give you the name, address, and phone number of the agency that provided the information." (Taken from A Summary of Your Rights Under the Fair Credit Reporting Act from the FTC)It cannot be spelled out any more clear than that. Also, as a company that conducts the backgroud check, that company is required to inform the person that the check was conducted upon to notify that person.
Duties of users taking adverse actions (emphasis mine) on the basis of information contained in consumer reports. If any person takes any adverse action with respect to any consumer that is based in whole or in part on any information contained in a consumer report, the person shall (1) provide oral, written, or electronic notice of the adverse action to the consumer;
Here is one of the key reasons - you have a right to dispute anything in the background check that may be negative.
"You have the right to dispute incomplete or inaccurate information. If you identify information in your file that is incomplete or inaccurate, and report it to the consumerThe bottom line is - if you have been turned down for employment, rental of property, or anything else because of background check - you have a right to know.