I suspect this isn’t about the copyright issues; ffmpeg also has potential patent issues. Some of the ffmpeg decoders probably infringe on active patents, so bundling ffmpeg with your application might require paying for a patent license. The patent license issues are explained at the bottom if this page: https://www.ffmpeg.org/legal.html
I assume Canonical has some sort of agreement in place to license some of these patents. There are a bunch of snaps created by canonical employees which contain ffmpeg, so I’m not sure how that works legally. Maybe @popey or @Wimpress can comment on this since they worked on the ffmpeg snap?
The browsers use the chromium-ffmpeg content snap which provides libffmpeg.so. @oSoMoN might be able to give more information about whether this can only be used by browsers or also by other applications.