This is a truism. I'm not saying it's true, neccessarily, since that would be bad and judgemental of me and at some point bound to bite me in the ass, and god knows, we don't want that. But why is it a truism, anyhow?
I never really went through the Big Copyright phase, myself, even in the days when I was far worse than most of the art on the net, largely because it didn't occur to me, and nobody ever tried stealing anything. These days, I'll pop a discreet web address in the corner, just so that people know where to find more if they find it somewhere un-accredited, but I only go to the lengths of do-not-alter statements when I don't hold the copyright myself, and even then, it's more of a token effort than anything else. I confess, for awhile the copyright statements on my website were fairly corny, but I did make them at least entertaining. (I was particularly proud of the ones in haiku format.)
My best guess would be that if you're good, you're well known enough that people are less likely to steal, or more likely to be noticed, and so you don't need such marks because people look at your art and go "Well, that's obviously an *insert Artspawn*" but that seems counterintuitive, since I'd hope that people would be more likely to steal good art. (By steal, I mean "claim as one's own" since unaccredited display is usually well meaning and nothing to get one's panties in a knot over.) Or do people just steal crappy art a lot? Is there any other simple, logical explanation, or is this phenomenon just one of those things that make you go Hmmmmmm? Is the truism even correct?
This was inspired by yet another Art-Vigilantes-Taking-Applications thread at VCL--I'd link to it, but the misspellings and grammatical errors made even my relatively lenient brain twitch and flop like a fish that had come out of water to watch that one episode of Pokemon, and I'd as soon spare y'all. (And I live with a guy who spells "cat food" with a U and an umlaut.)