I know one person has successfully decoded it, and that it is just the header/index which is encrypted, but said person has chosen not to make the discovery public (presumably for legal reasons). (No, it's not me. Like I could figure that out???)

By that definition, then the entire free community been supporting TSR since day one of TS1, so it's a bit silly to start complaining about it now.

The free community is just that. Free. We don't try and withhold information or knowledge or specifications or tools in order to advance our own position. We publish it, we share it, we have wikis for it, we write tutorials for it. Because our information is freely available, TSR has complete access to all of this information without any catches. They always have, and they always will. The difference in this scenario is that TSR are adding their own findings to that general, "free" set of community knowledge. Paysite politics aside, that is something of a show of faith from them.

Riverview, and all DLC from the EA site, is in a compressed DBPP package file format. This is most likely to prevent users from simply cloning it, renaming it, and putting it up for download. None of the existing package tools will currently be able to view or edit these.