It isn't "de facto" legal if the owner decides to pursue it or not.

The problem is the lack of knowledge of rights associated with intellectual property.

Many people make their bulletin board flyers by searching a related term on Google Images and pasting it into their document, without any thought to the licensing or knowledge that they might need a license. Despite that they are not likely to face legal consequences, it doesn't erase the copyright holders' rights to their image -- even if the image is very popular and many people unwittingly infringe on the copyright every day.

TW's claim was invalid here because they did not own the full rights to "Happy Birthday to You", not because of the popularity of the song.