Yes it is. If an artist draws something, they have author's rights over that specifical piece of art, and make money from it, UNLESS the original creator of the character asks for the piece of fanart to be removed, because, in priorities, their copyright for the character and all its media matters more than the copyright for the piece of fanart. But no anime does that, because the anime industry is small, poor and don't want too much trouble.
To have a copyright over a piece of art, I think it's not that difficult. Some sites like Instagram will defend whatever artist that gets their art stolen (like those typical anime pages that are only reposts with 'credits to the author!') Some other sites will require something more formal and that would cost money, I think.
But wjat Hydaria does is that they hire lawyers and people to check on the internet in search of reposts of their pieces, like in 4chan. So, it's both about money cost and how much trouble you're willing to go through. Some artists like Mazjojo most surely have the money, but not the dedication, and some other artists most definitely have the dedication, but not the money.
And this is one of the reasons Hydaria gets hate on these threads. Because of how "greedy" they are. To put it somehow.