I guess it has nothing to do with playing police, but there are some minimum standards, that are crucial for steemit to succeed. If it turns into a platform, where people get rewarded for the work they stole from others, it would really harm the whole community. For that reason there is a group called "steemcleaners", that you should mention, whenever you see this kind of abuse. They will take care of it.
Posting some artwork you like and adding your own inside while linking to the source is absolutely ok, though.