Honestly, while I do get the OP's point, I don't think this is quite as poorly played as it seems at first. Having a giant spoiler warning at the beginning of an MMO expansion trailer says "wait, this MMO has a story worth spoiling?", and then the short clips they show demonstrate that things get serious. If anything, it will probably help generate some interest so people can catch up on the story between now and when the game releases.

I think S-E is treating the expansion like a movie sequel, so the trailer is both to build hype for the sequel, but also to prompt people to catch up before it airs. And for people who definitely don't want the spoilers, there'll be plenty of other promo clips showing up in time, I'm sure.