Two entirely different things. What you are talking about is FCRA and the employer is required to provide a copy of the Background/credit check to the employee if the results preclude the employer with following through with the offer. It has nothing to do with the offer letter itself.I'll take a look a little later online and post a link. I have the law in my HR book (bible)though and where I've worked we have always had to provide in writing to someone why an offer was rescinded. I'm pretty sure you are legally required. As you probably know this is required for example when a background check comes back and a Company refuses or rescinds an offer based on the results.
You can go checking your "HR Bible" and I guarentee that what you have is not even close to what the subject is here.