I added a bullet a few days ago to that specific rule, since it's the only one that has this particular requirement.
In addition to what animuson said in the comments (that we don't want to maintain site-specific pages unless absolutely necessary), there are a few other practical reasons why I went with this option:
This exception isn't specific to MSE; it applies to all meta sites. But other meta sites don't have their own help center; they rely on the main site's version. So there's no possibility to override them even if we really, really wanted to maintain 170 versions of this topic.
Conceptually, child metas are a part of their parent site (hence the shared help center). So treating them specially here is just confusing.
The notes that contain the phrase "all sites" apply to the rule sets - in this case plural - immediately following them. Only one of those rule sets treats meta questions specially.
The only reason this help center page exists at all is so that we have something to link to from the note below questions that have been automatically deleted. As such, the vast, vast majority of people reading it are those who've participated on a question that has definitely qualified for one or more of the rules. It's therefore important that they be given a convenient means of identifying the specific criteria that led to their question's removal.
The population of people reading that page in hopes of identifying why their question hasn't been deleted, OTOH, currently consists of you.