Oh my, I have never heard of this before! And would never touch a H bag without a stamp.
I feel the same, I see no valid reason for Hermes NOT to stamp a bag. Yes there may be oversights where a bag might slip through the net and not get stamped by accident. I just dont see it happening intentionally.
Hermes are proud of their products, and not putting their name on the bag "because it is too perfect" doesn't make sense on any level.