The opus software implements several ideas covered by patents. The software is open source, but using it and selling it is potentially encumbered in patents.

In this case, they seem happy enough for people to keep using it under the open source license (I'm not sure the patent pool has any direct power over the software), and seemed more focused on where the money is: people building this into hardware.

I'm with you though. They shouldn't have described it as royalty-free.