But I don't remember promotional shots and commercials ever before being considered spoilers. Yes, the kiss comes from a future episode that hasn't been seen and no one has any clue about. But if you look at it that way, it was a spoiler to all four people who were watching BG at the time ;) It was a spoiler to everyone in the US. (Personally, I think the problem here is that it aired in the US first. If a commercial showing a Sam/Jack kiss had aired in the UK, the exact same thing would have happened.)
So what's the spoiler? Sam and Jack may (assuming the clip actually made it into an episode and wasn't cut, not knowing exactly how Skiffy's ad department opperates) kiss this season. We don't know how, we don't know why. We just know it might happen. And it's simply too big a piece of news to keep behind cuts until it happens.
As Jo said, it's one of the perils of online fandom.