Why would that be illegal in the USA? It's not slander or libel if it's true.
There are plenty of good reasons for companies to not honestly describe bad behavior of former employees, but I don't see that one of them is that it's illegal.
I think a number of people misunderstand that the reason why a lot of companies have a "Don't talk about why someone left" de-facto policy is because if there's any ambiguity it's probably not worth the cost of having to defend yourself from a possible libel lawsuit, and not that it's against any law.
The cost/benefit of that changes if there's no ambiguity, evidence of truth is after all a good defense against libel charges (in most sane jurisdictions...), and if the reason is "big" enough they may feel obliged to make it known.
Defamation that questions someone's ability to do their profession is in a special class. You don't want to be the slightest bit wrong about anything in that, i.e. were they misquoted or overheard instead of talking directly to a reporter? Might some employer somewhere who would hire them view any such distinction as different?
Similarly, they leaked data but didn't or he lied and they were entering the phone market? Details wrong could be a long civil case, all the details right could still be a long case.. The only reason to engage in that kind of behavior is to sabotage yourself.
There are plenty of good reasons for companies to not honestly describe bad behavior of former employees, but I don't see that one of them is that it's illegal.