In reverse order to your post, I will concede that some unions have become too entrenched, but employers have rights too, and if they have an actual documented case against an employee, they can fire them.
Jumping back; They weren't trying to shift blame onto a contract, they were nearly stating a fact: Unions usually have an obligation to defend everyone as part of their contract. although were mistaken in saying that it was in a contract, when it is actually part of founding principals of unions -- As the NLRA defines a labor organization as a group "[...]in which employees participate and which exists for the purpose, in whole or in part, of dealing with employers concerning grievances, labor disputes, wages, rates of pay, hours of employment, or conditions of work."
dealing with employers concerning grievances, labor disputes -- it is the entire purpose unions were formed. You replied with a remark to the effect that companies had no say.
Maybe I read your tone wrong, but I read negative, and decided to go with a snotty reply, "who cares what they ask for", which could have been thought out better -- but is the whole point of this in the first place: Unions don't support bad behavior, or reprehensible actions as you say, unions protect you through the process of being accused or actioned upon, despite the reason. They exist to handle disputes. Simply that.
I am curious which union you belonged to