Kudos to Humana! I received a phone call from my Humana contact, who had graciously taken the time, and made the effort, to track down the corporate reasoning behind the HSA discount/non-discount issue.
I'll present his answer here, without embellishment or comment:
"It's contractual. That is, Humana [ed: and, presumably, the other carriers] contract with providers on behalf of their insureds. As part of the negotiations, the carriers agree to re-price only those services which are covered under the contract. This is to help the providers recoup some of their income losses on the re-priced claims.
The principle is that it's only fair that if the carrier's not paying for a given service, the provider shouldn't have to, either (in the form of a discount)."
We'll have our own response to this shortly.