Aye, too bad I just can't bring myself to do it. ;p
Actually, I just recalled an incident in which a colleague accidentally sent a publication (think it might have been an annual report) for print, and had missed a couple of images that he forgot to relink after the full sized stock images were purchased, so the istock comp images were still used. Thankfully, he had already done some DI to remove the istock logo when presenting the drafts to the client, so the issue was pixelation due to low resolution rather than a big istock logo over the image. Still a problem, but a lot less embarrassing.
Bottom line, mistakes do happen due to negligence (lack of QC) & ignorance (untrained/uneducated), so I wouldn't assume straightaway that someone was trying to avoid paying usage fees especially if they didn't even bother to hide the watermark. Of course, it doesn't excuse the mistake.