The problem with patent licensing and open-source software is that patent license agreements are usually geared towards shipping physical items (such as DVD players, etc.) Commercial, proprietary software tries very much to behave like physical items (one single producer, building and shipping items to to customers) so patent terms like "20c per unit" work quite well.
Open-source, on the other hand, has a most unphysical habit of proliferating without the original creator's involvement. If Canonical promises to pay 20c per CD they ship, and 10% of those CDs are imaged and torrented and downloaded and burnt and re-imaged and re-shared all over the web, nobody knows how many "units" are out there, and there's no reasonable way Canonical (or Mozilla, or any other prospective licensee) can comply with any patent license more complicated than "do whatever you want".
Open-source, on the other hand, has a most unphysical habit of proliferating without the original creator's involvement. If Canonical promises to pay 20c per CD they ship, and 10% of those CDs are imaged and torrented and downloaded and burnt and re-imaged and re-shared all over the web, nobody knows how many "units" are out there, and there's no reasonable way Canonical (or Mozilla, or any other prospective licensee) can comply with any patent license more complicated than "do whatever you want".