That doesn't seem to be a problem in this case? Telling spammers they are blocked due to copyrighted images trains them not to upload copyrighted images. Win-win.
picking up copyrighted images is another indicator that user X is a spammer, providing that info would eliminate the signal
Well, this is the essence of discrimination and we wouldn't tolerate it for a whole range of indicators (you're black, gay, if a particular race, etc etc). My guess is the real reason they won't tell people is that they would end up in court pretty quick.
so, in you mind, detecting copyrighted images and using that as a metric to detect spammers is discrimination? Are antivirus programs discriminating too??
I bet you indent your code in an inclusive way
From a definitional point of view yes. Using an attribute to place someone in a class and then making decisions on a class basis without actual evidence they possess the other attributes of the class is discriminatory behavior.