I think you're mixing up two different things there. And although it feels kinda obvious to me, the difference is actually paper-thin, so it's really tough to explain. But I'll try.
When you say a story is bad, all you do is giving out a personal impression, an evaluation that is clearly identified as such. Thus, it's up to the recepient to buy into it or not, and to which extent. All you can change is someone else's impression of a story.
By that logic, even a blatant lie would count as a spoiler.
With an actual spoiler, like "___ person dies", you're giving out "real" information. Even if the recepient doesn't believe you, the impact of the reveal still lessens. The only way to resist it is to forget the spoiler before it becomes relevant. By revealing something that's is not yet supposed to be known, you actively mess with the author's intention as he's carefully guiding his audience down the path he has laid out. Basically, you're interfering with the story itself.
Sure, it depends on the author's skill whether or not he can get the audience to think and feel what he wants them to, so a bad author may not get his message across and even falsify it, but that's a different issue. Again, if you convince the reader that a story is "bad", you make the author's job harder, but if you interfere the flow of information, you make the job impossible, UNLESS his information management is messed up and you somehow "fix" it with your interference.
I know I'm sitting on my high horse here, but I think that, with so many grading sites around, people so focused on whether something is "good" or "bad" that they're getting tunnel vision. Right now, you're only concerned about whether the viewer/reader will find a story "good" or "bad" depending on whether or not you say something to them. But that's just the last part, a subjective evaluation of their experience. On the other hand, by revealing "real" information to them, you change the experience itself. The meaning of a scene may change. The things that were supposed to be conveyed may not get conveyed. You see, what a certain scene, development etc. makes you feel are things like joy, sadness, disgust, etc. but not "it was good" and "it was bad". As I said, that's only the evaluation of what it makes you feel. By influencing that evaluation, all you change is the intensity of what is felt, but not its nature.
...well, this doesn't sound very convincing, but I hope I could get my point across, even if just by a little bit.