You are not required to do anything, but if nobody does, then Steemit may just turn to a place where you will not be willing to come at all as the kind of content you will find will not be worth checking out... The least thing everyone can do is not to encourage this kind of behaviour by not voting for such users and posts.
Let us say you are a witness fo a crime, should you:
a) Do nothing and just let it go watching idly from the sides.
b) Go all out and try to prevent the crime from happening yourself.
c) Call the police and let them do their job and handle things professionally.
I guess there are people that can do any of the three options above. Wen we relate that to Steemit and plagiarism as being the crime... just that going for A may not be the best thing to do when compared to the other two options and nobody says you should go for B either when there is the option C also available.