Blue Mage is a masterpiece of game design.

[Seraphor]

To a lot of people, the shout you refer to as 'spoiling' is as helpful as the other shout-types you mentioned. It's very subjective. Therefore it's up to every person to decide what they prefer - to get 'spoiled' and benefit from the rest of those boons, or avoid spoilers and miss out on everything else. It's not one's job to censor your chat channel in this game but yours.

This is true, but it doesn't make it any less the fault of the person shouting the spoiler.
As you must weigh up the risk/benefits of hiding shout/yell, when you shout a spoiler you weigh up the risk/benefits of doing so.
You can always Tell the person asking.

Being your fault doesn't mean you need to be censored or that it should be against the rules, but those who are spoiled are justified in being annoyed at you.