Before anyone wants to be contrarian about this: no one is going to stop you from using a stranger's weeb digital art as your profile pic. It is not illegal to use someone else's art for your profile pic without permission and no one is saying it's should be.
The point of this post is that this is an extreme overreaction to a conversation that concludes with 'pwease ask this other person for permission, the art is of their character design not mine 🥺' in a niche art community that operates on an honor system for the sake of being polite to one another.
just because it isn't frequent or high profile doesn't mean it's legal. It's just usually a mostly harmless use so people/companies don't often pursue it. In some cases it could maybe, barely qualify as fair use even. But in this case, the picture being used isn't even really incidental to the work that's being sold, it is the work that's been sold, for exclusive use, so it being used without a license is supplanting the market for the original. Anyhow the US copyright system is awful