copyrights and citations

In a world where we are warned not to plagiarize I am constantly confused with how fast this simple rule is forgotten on the web. Last week we learned that when making a web site you MUST give credit to any image you use unless you yourself took it. Common sense to writers and artists. And yet blogs like Tumblr exist where credit is never given and when it is, some annoying person decides to erase it because the artists name is just far too gross for their beloved tumblr's layout. How is this allowed? How are we allowed to share music and art without giving credit? Where have the rules gone and why is this grey area not taken into account? I feel like I am the one of few, linking back to artists websites and citing the creators.