When an NCO arrives at a new unit, where does it say they do not have to provide previous evaluations or place them in counseling packets?

During my initial counseling with my NCOs, I usually ask for on the front side of their last three NCOERs, as this will help us both develop an understanding of previous assignments, primary and additional duties, etc. The back side of the NCOER isn’t necessary, but it can serve as a good professional development tool further down the line when discussing professional growth, good NCOER writing techniques, and other things.

SSG Doug Mckee,
After being a Personnel Officer for almost 7yrs I’ve routinely found myself in a very similar situation. As a young AG Officer learning and perfecting my craft I can tell you that I've made some mistakes that I learned from and told myself I would never allow to happen again. One of those mistakes is the dilemma you have provided us today with this question. It’s very common for Commanders/CSMs to express the notion of past evaluation requirements for things including, but not limited to: In-briefs, TOE placement, unit rating scheme, etc.. you get the point. However, it’s so important for you as the provider of personnel services and records management for the Soldiers in your unit, to safeguard their privacy and keep the integrity of what we do intact. SSG John Carpenter is right in that Army Regulation 623-2, paragraph 1-12 clearly states that and pretty much sets the conditions. You can also find quite a bit of info regarding this privacy in Army Regulation 600-8-104, but the simple fact is that these regulations aren’t needed to express this privacy and or needed to produce some sort of proof for your chain of command. Here’s the skinny, you, even as the HR NCOIC (an assumption based on the question) can’t even access that information. Furthermore, your brigade, your division, or even Corps level do NOT have access to view and or pull individual evaluations for anyone outside of themselves. The only way this is possible is by being granted access from the OMPF custodian from HRC (Commanding General, PERSCOM) and or the expressed consent drafted, signed and forwarded to the OMPF custodian from the individual you need access for. So, with all that being said, all you have to tell your chain of command is this, if you were authorized to view or pull this document you would be, but you are not.

Where does it say they do? Transferring units is supposed to be a fresh start. Otherwise no one would ever recover from that stupid mistake they made when they were young.

The only thing would you be able to see would probable be their annual, since it is a permanent record. Counseling form are not permanent, and should be destroyed upon PCS.

The closest thing I found is in the DA Pam 623-3 NCOERs paragraph 1-5. It basically states where NCOERs will be maintained. Basically all the gaining unit needs to know to begin the NCOER process is the through date of the last NCOER. You can get the previous assignment data from the ERB.
1–5. Privacy Act statement
c. Routine use. Evaluation reports will be maintained in the rated Soldier’s Army Military Human Resource Record (AMHRR). A copy will be given directly to the rated Soldier or sent to a forwarding address. The DOD blanket routine uses may apply to this collection.
Note: Completed forms contain personnel identifiable information and require special handling.

Offer a compromise. Ask the NCOs in question if they have any issues providing just page 1. This would prevent you from seeing 99.9% of anything from the previous unit that could be potentially harmful to their fresh start at your unit. More importantly, it enables you to verify the newly arrived NCO received an NCOER from the losing unit. Of course, you could always check ERS. Have you considered having the inprocessing NCO provide an ERS print off showing the status of all their NCOERs?

All the gaining unit supervisor is entitled to is that the NCO received a rating prior to PCS. If they have a bad NCOER you will likely learn that as they usually try to gain support to appeal it, often futile. There is no reason for you to know how they were rated, million things can affect a rating. Just perform your duties and they perform theirs. Thank you for your service.