Furthermore, the most commonly broken rule is subjective too. Who decides what is low value or uninteresting? Why enforce the spam rule but not the trolling rule?
The way I see it trolling requires intent and moderators are reluctant to judge intent.
Low value post can still be low value even if the author didn't intend for it to be low value and that's easier to judge objectively. Trolling is only trolling if the troll intended to troll and not e.g. to express their opinion in an inflammatory way.