One is not a "whistleblower" in the legal sense for reporting something to HR. Rather, the law only considers a person to be a "whistleblower" when the person reports a suspect violation of law to a government agency. Because the facts you describe do not suggest that any employee filed a complaint with an outside government agency, this is not a "whistleblower" matter.
As you previously indicated, the employees are "at will". This means that you could quit your job at any time and there's nothing your employer could do about it.
That also means the reverse is true: your employer could terminate you at any time and for any reason (so long as it wasn't because of your race, age, gender, disability, national origin, etc.) and there isn't a darn thing you can do about it.
An employer may dismiss an employee at any time for any reason; a good reason, a bad reason, or for no reason at all.
Now, does that seem to be unfair or unjust? You bet it does. However, it is not illegal. That is the very nature of "at-will" employment. You serve "at the will" (or whim?) of the employer.
Of course different people can disagree as to whether or not the punishment fit the alleged wrong in the situation of your former employees. But, because they're an at-will employee, the employer doesn't have to even go there.
They could have been fired because the employer didn't like which hand they used to lift their coffee cup.
They could have been fired because the employer didn't like the shade of green socks the employee wore to work. They could do this because the employees were "at-will".
Good luck and best wishes for better days ahead.