CNN had a story on this and you'd think the SIOP plans for nuclear war had been uncovered. A former DHS inspector general said he thought criminal charges might be in order over this.
Having read the document, I am genuinely puzzled why this was deemed to be SSI to begin with. If anything, it should have been in the public domain from inception; it would cut down on innumerable misunderstandings and misperceptions at checkpoints. I'm open to hearing another viewpoint, but it seems to me that if TSA is actually doing its job, having this public does NOTHING to lower security.
You are 100% correct.
Of course, if it's hidden then it allows the TSA to do pretty much anything it wants and claim "SOP" yet have the claim be unverifiable.
If the SOP were truly public domain, every member of the public would be another set of eyes making sure that the TSA is doing its job properly. That is likely what TSA management fears the most.