Fair use requires you to analyze or comment on the work or transform the original work in some way, non-commercial use isn’t enough on its own.
The real reason it’s “ok” is that it’s unlikely the owner would be able to prove damages from someone using their image as an avatar so it’s not worth taking anyone to court over it because all they would get is the judge ordering them to stop and a C&D is cheaper if the owner really cares about stopping you.
Fair use requires you to analyze or comment on the work or transform the original work in some way, non-commercial use isn’t enough on its own.
The real reason it’s “ok” is that it’s unlikely the owner would be able to prove damages from someone using their image as an avatar so it’s not worth taking anyone to court over it because all they would get is the judge ordering them to stop and a C&D is cheaper if the owner really cares about stopping you.