We’re talking about Android, unrar doesn’t have anything to do with this really.
The entire topic is about RAR archive support on Android, so of course the freely available source code of unrar, released by the RAR developer himself, has absolutely to do with everything here.
RAR is and will continue to be a proprietary format with an owner who can seek royalties.
Nope, unrar's source code is free, released by RAR's developer.
It’s like saying Google should stop licensing MPEG because ffmpeg exists—it simply doesn’t work like that
Nope, it absolutely isn't like that. You just have no clue at all.
Unrar source may be used in any software to handle RAR archives
without limitations free of charge, but cannot be used to re-create
the RAR compression algorithm, which is proprietary. Distribution
of modified Unrar source in separate form or as a part of other
software is permitted, provided that it is clearly stated in
the documentation and source comments that the code may not be used
to develop a RAR (WinRAR) compatible archiver.
It's not FOSS, given that it comes with the provision that no RAR compressor can be created based on unrar source code but for browsing and extracting RAR archives, the unrar source code as is is absolutely fine.