With the large number of posts it is becoming quite a task to weed out plagiarism. Unfortunately as word of mouth start to spread I am afraid Steemit will start to be abused as a way to make easy money.
Flagging a post for plagiarism is a start when they are caught but what about the many others who are not and walks away depleting the reward pool for all.
I am sure the Steemit team is actively working on this problem to more easily detect cases of plagiarism more efficiently and hopefully a solution can be found before Steemit explodes on the scene.