i'd say its best to check the user's terms and conditions for their art. for example, some artists allow copying for practice but not posting, and others ban it outright. generally tho, its not good to copy artists' work regardless, so imo i would class it as art theft.
I wouldn't say it's art theft if the copier isn't claiming it as their own. That's why fan art exists - they aren't doing anything wrong. The problems lie with putting that copied art somewhere the original artist would not approve of or accepting credit for the creation of the characters. But the copying for practice or just for fun is not stealing in and of itself. Or else why is there so much fan art of popular characters? I don't think anyone asked the original illustrators for Disney if they could draw Ariel or Belle, etc. They just aren't allowed to sell it as their own without a license from the owners (I believe). Well, this is just my opinion anyway. Who knows..
Fanart = / = copying
Fanart is drawing art of something you like, not directly copying it