There are a lot of things regarding organizing that are protected by federal law (see: https://www.dol.gov/olms/regs/compliance/EmployeeRightsPoster11x17_Final.pdf), but you really need to have a solid case with conclusive evidence that you were mistreated/fired specifically because of your role in organizing, which is very difficult to do. For example, a disgruntled TM at my store who had merely mentioned organizing in a venting session to a fellow TM was once taken out into the alley connected to our receiving dock by our STL who then proceeded to demand names of all other TMs involved in the "conspiracy."
The STL backed down when the TM started to question his tactics, but either way, no cameras caught the incident (even if they did, there wouldn't be any audio), and there was no "official" action taken against the TM (write up, note to file, etc.), so what is there to do about it? Everybody knows that PBS doesn't honor any sort of confidentiality, let alone actually follow up on claims of leadership misconduct, and that the TM tip line is more or less the same. But now, everybody at my store also knows now that if you mention organizing within earshot of a STL suck up, you are gonna get taken outside TO A F***ING ALLEY and get a stern talking to.