I guess no law prevents anyone from saying that someone is, or isn’t , eligible for rehire. It may be that some State has law that insulates a former employer from liability for answering that question. I don’t know what HR textbooks might say today. I’m often a bit out of date .
It could be more a question of what lawyers think is least risky. Many employees have some grievance against the employer. Often good ones. Or they might be potential whistleblowers. It’s best to let them quit and have them leave on the best possible terms thinking they are welcome back. If HR then tells potential employers the guy is not eligible for rehire,and he’ll figure that out if he cannot find a job, he’s apt to file a claim or report something. Why risk that?
The less said the better.