Wikipedia attempts to justify the spoiler convention (at least, that's what I think they're doing) by explaining that "Not all visitors will recognize the site as an encyclopedia, which should strive first to inform, spoilers or not."

With my webmaster hat on, I suppose they're making a valid point (though I think it's being taken to silly extremes). With my everyday hat, though, I'm having a big attack of YOU'D THINK YOU'D THINK.