I love the conspiracy theories.
Could it be the SSPX wants control of the SSPX's own intellectual property?
The publisher has explained what the SSPX demanded as regards the content of the sermons.
This isn't about the SSPX getting a cut of the proceeds of the sale - this is about suppressing Archbishop Lefebvre's sermons. Whatever their "rights" - there's no possible good motive for this - it stinks.
The SSPX may claim to "own" Archbishop Lefebvre's sermons. I suppose they would claim to own Catholicism and the Bible if they could get away with it.