We direct all questions specific to one site to that site's own meta, even if someone may not be able to post there. This rule does seem kind of obtuse, but there's a reason for it.
In order to ensure that all meta site communities can handle questions appropriately, meta questions are separated into two categories, to be asked in different places:
Meta Stack Exchange is for questions that are related to the entire Stack Exchange network as a whole. For example, bug reports or feature requests for the Stack Exchange Q&A engine, questions how a particular aspect of the engine works, and questions or discussions regarding a general governing principle for the network sites, may be asked here. This site is also used to disseminate announcements related to all sites (e.g. a new feature implemented on all sites), as questions asked here can be given a special tag that links them on all sites in the network.
The general community that participates here is generally separate from the community that participates in a particular Q&A site. As such, we may not be familiar with the particulars of a site's rules, or why a particular post was handled a certain way.
As this site is not tightly linked to a single Q&A site in the network, reputation and privileges are separate here. Also, because of that, it cannot check to see if you've earned 5 reputation on another site, so this site allows open posting by all users with an account. This does not mean, however, that users who are unable to post on the site's own meta should necessarily make their post here in all cases.
Per-site meta sites (or child meta sites) are for questions only relevant to one site in our network. For example, requests to reopen or undelete posts, discussions whether or not a tag should be removed or whether a particular type of question is within the site's scope or not, and announcements related to that site (e.g. moderator elections or resignations) belong on the site's own, designated meta site.
The community that participates in a site's meta is the same community that participates in the site itself, and so is much more familiar with things relating to the site in question, and is thus more knowledgeable to tell you the specifics of the site's rules or why a specific post was handled a certain way.
Because the majority of discussions taking place on per-site metas are about the site's governance (the latter two example categories above), we require that all users have at least a little experience on the site itself before being allowed to post there. However, we understand that this ends up leaving out completely new users who'd want to better understand why specific actions were taken on those posts, or appeal such actions, so there's a system whereby new users can ask questions about their own posts without needing to earn reputation.
tl;dr: Site-specific questions are considered off-topic here because the (mostly separate) community here is often ill-equipped to handle them, and such requests are much more easily handled on the site's own metas because the community participating in those sites is the same as the one on the site itself. Because most discussions on site metas are about the site's governance, we require a small amount of reputation to post on those sites, but there is a system that allows new users to bypass this limit if they're asking about their own post specifically.
There are some narrow exceptions to the site-specific rule: for example, questions or requests about the international sites written in the English language may be asked here (as the international site metas may only accept questions in their own language), and questions that pertain to Meta Stack Exchange itself can be asked here (i.e. Meta.SE also serves as its own per-site meta). This site is also home to discussions that aren't about the network as a whole per se, but relate to two or more specific sites in the network (e.g. questions about MathJax, the math markup engine, which is only enabled on a few sites). Also, a small number of per-site metas allow open posting by new users.
