It's not a grey area lol, if you murder someone and no one notices it's still a crime.
Do not equate murder and copyright infringement. They are different ethically, morally, and under the law. Sociopathic kids these days...
Seriously though, in general copyright infringement only happens when there is a complaint. I have hundreds of photos online that are used daily and many times without adhering to the terms of my license. To actually litigate every one of them would be a huge burden for minimal payout. And as in most cases no one is making appreciable money (someone just tumblr's something or shares it on facebook or uses it as an eyecatch for an article/blogpost without advertising) I don't see the point.