First, seeing another account that does the same thing doesn't mean it's not against the rules. That would be like saying "I saw a guy break into a car the other day and he got away with it, I guess it must be legal."
Second, if said account you saw were legitimate it would have had to take certain steps to make it so, none of which have been done by the user who posted this picture of Ahri.
You're not allowed to 'promote' work without permission. There are some accounts on here that post images on behalf of artists that work for their company. Other times artists will give permission to for somebody else to post their work (giving them credit) if they don't have any interest in creating a deviantart account but want to get their name out there a little bit. The third time others' work can be used is if it's labeled as a stock photo or stock art, in such cases permission is implied but the creators usually ask you to give them credit so that their page can be seen as well.
As I said before, deviantart is not tumblr, it's not reddit, and it's not facebook. It's meant for people to promote, share, and get help with their own, originally created artwork, or artwork created by someone who asked them to post it. It isn't a website for you to post random images you found on the internet that you thought were 'cool.' Such use is technically illegal here, and even though it still happens, most deviations of this nature get taken down within the week they're submitted.