His mistake was not citing the original work. People wouldn't have a problem with seeing a work reproduced, especially if it's as witty as this, as long as it didn't appear as though the duplicate was seeming to be flogged of as a new work.
If you're reading this, Johnny Bigtime, you probably want to do something about that. XKCD is a very popular site; people will tend to get the wrong idea.
boy, all you people need to relax maybe the guy messed up. but you don't have to hang him. like you never made a mistake. take a fricking chill pill people if that is all it takes to send you on a spastic fit you will probably die at a very young age or you need to get a life!!!!!!!!!!!!!!!!!