Just to be clear, AEA members do not need to be given any notice - depending on the reason - and can be let go immediately, just paid out. "Just Cause" can be a variety of reasons (lateness, not abiding by drug and alcohol policy, missed rehearsals, shows, breach of contract) - but often it's general management making the decision, but you can be just let go for no reason, and paid out as well - simply, we don't like you, here's is your contractual out, have a nice day.
I am not sure they "were wrong" in not giving the reason, maybe not cool, but right and wrong is in the eye of the beholder - I think it has a lot to do with that letter said and what the reason is. I find it hard to believe that these two people were doing great and then they were just let go. Obviously, the ASM there was some sort of reason. The actor, who knows. And the gossip seems to take care and make it bigger then it.
What was wrong is that management didn't chose to take the opportunity and discuss the issue with the remaining company - when you fire an actor, there is always some sort of backlash - so it's best for management to discuss.