Most people are not obsessed monomaniacs with room for only one interest in their lives. That means that most Linux users are interested in things other than free software, so many will choose to “dress up” their computers in ways that showcase their other interests, and may use material that is not under copyleft licenses to do so. If this causes you anxiety or confusion, you may want to speak to a mental health professional. Seriously.
My primary icon theme and widget style are 20+ years old and not flat in the least. You can still have that look and feel on a real computer if you want it (but you may have to compromise elsewhere or do some extra work). On phones, all bets are off.
Dunno what your issue with that icon pack is, but I’d bet there’s a good chance it can be solved with a few file renames or symlinks if you care enough to bother.