As a retired postal employee I can understand your frustration and disappointment. During my years working for the USPS, I saw some pretty bad mangers. Many knew little about their job, and were unwilling to learn, and never forced to learn the job. Others knew their job, but just didnt care enough to do it correctly, and were covered for by management levels above them. Whenever an employee violates the law or the regulations, it is up to the manager to deal with it appropriately, but some see doing their job as too much work, so they just let things go and blame the union for the manager not doing their job.
Another misconception in the USPS is that employees cant be fired. That is not true, as employees are fired all the time, sometimes for some pretty lame reasons, such as, an employee can be fired for letting a dog bite them.
In regard to the union "protecting" employees, unions are required to do so under a law of "the right to fair representation", where the union can be held liable if they dont provide and "adequate defense" of the carrier even if they know the carrier has done something wrong. It is pretty much the same standard lawyers are held to when representing a client.
I hope things work out better with the USPS in Colorado Springs.