It's easy really. The photographer has often spent a lot of time and money creating the work. All businesses have overheads, and photography is no different. There are free sites like Pixabay where users can get images that are copyright free.
I believe that steemcleaners should class any source other than the free sites as plagiarism. (It's copyright infringement too, but stopping the plagiarism would stop the copyright infringement.)
Pixabay is a wonderful site actually. Perhaps promoting more awareness about the site (and others) and how you can get copyright free images would help as well, at least until @steemcleaners can workout a solution.
I think that many people just think you can pull an image from the net and not worry. Some knowing it is infringement and some likely not knowing as well.