Oooh! This sounds like an appetizing opportunity for a hungry lawyer somewhere. Resigning without any notice is just as legal as firing without notice. If UAL is effectively blackballing these employees, they might be in for quite a surprise when the process servers start showing up at HQ.
Yes, resigning without notice is legal.
To clarify, this was only done for people who gave absolutely no notice to UA that they were not going to return, and gave every indication that they would be back on a certain date, but just never showed up or bothered to explain.
So we are really talking about job abandonment here -- but that is a form of resignation and is also "legal" in that an employee breaks no law for quitting with no notice (though s/he may break company policy).
It is also legal for a company to decide you are not eligible for rehire, especially for job abandonment, and even more especially if you lie to the company by, for example, saying you will show up for training on a certain date, and then do not.
I don't see any cause of action for a lawsuit.
The business owners I know won't EVER comment on anything relating to performance. They only confirm the period of employment. And they especially would not be stupid enough to hinder someone's future employment opportunity by implying anything negative and expose themselves to lawsuits (even to the extent that they won't mention employees who were fired after being convicted of a crime).
Resign? And have the company slander you? 🙄
Your legal knowledge is weak.
First, nothing about "performance" is being given out in this case.
Second, it is completely acceptable -- and is standard practice -- for a company to verify dates of employment and confirm eligibility for rehire.
Third, in most -- probably all -- states, there would be no cause of action for giving out this information. Many states now even have statutes specifically protecting an employer giving out information like this.
Fourth, truth is an absolute defense to defamation. So if you are indeed "ineligible for rehire" at Company XYZ, there is no defamation (libel or slander) if Company XYZ says that, because it is true.