Because it's rude, of course. How would you like it if I blurted out "AERIS DIES" back in 1997 when
FF7 was new and happening? Or, say, the big plot twist to
Harry Potter VI (just bear with me, if you don't read those; otherwise you probably know exactly what I mean)? Hell, I wouldn't DARE run up to somebody waiting in line for
The Empire Strikes Back, when it first came out in 1980, and scream out "VADER IS LUKE'S FATHER!" - needless to say, I'd probably be much worse for the wear if I had (although since I was roughly -8 years old at the time, I don't have much to worry about in that regard).
Now, granted, you probably wouldn't know what it's a spoiler for from just the name (and I don't care to exacerbate things, so I'm not going to say what it's a spoiler from - I think we both know, anyway)... but still, spoilers are spoilers, and it's common netiquette to not post them without spoiler tags, or "(spoilers)" or something else. I mean, really. This is something you'd figure out after just a few days of browsing a forum - any forum - particularly when it involves something relatively new.
That's all I'll say on that matter. It's not like I don't already know of the spoiler, anyway, so it's not as big a deal as any of the plot twists I mentioned in the first paragraph - but still, now you know.
And knowing is half the battle!