Dear @juliank, I admire you by the way you place this theme, so negative and the main damaging factor for a photographer, in such a positive and encouraging way, calling for a reflection and for self-improvement by those who commit plagiarism. 
Plagiarism is an offense, it's against the law. 
I'm not indulgent with plagiarists, I myself have found out some of my photos being plagiarized by others and try to take immediate measures. 
But how many more I can't find out? I seldom use a watermark! And in the Internet is impossible to trace them all.  
We, the photographers, study hard, work hard and make a lot of investment in gear so our work can be worthwhile. 
I have to admit I was surprised by the tone of your post, in a positive way. 
I wouldn't be so swift. I would be furious! Cheers and take care!