Okay, my point was the other way: I assume it's not obvious that it is okay to give out the secret key. Indeed, if you look at the description of NaCl, the security model section (http://nacl.cr.yp.to/box.html) and the referenced paper do not claim that the message stays secret in a situation where the senders's secret key is known, even if the recipient's secret key is not.

Still, publishing the sender's key seems to be for that NaCl scheme, and everything else would surprise me, but I haven't looked at it in detail...