The short version
Citations are deep links by standard. Kindred prefers a link to the specific document a claim drew on, not a link to the home page of the site it lives on. A bare homepage is citable only when the homepage itself is the source, and a link that reaches only a site's homepage is labeled honestly as such.
Why it matters
A homepage link looks like a citation but does not do the work of one. You cannot confirm a specific claim against a front page that changes daily and may not even mention the material in question. Insisting on the deepest available link keeps a citation honest: it points at something you can actually read and weigh.
When a homepage is the right link
Sometimes a homepage genuinely is the source: a claim about an organization's own front-page statement, for example. In that case the homepage is the correct citation. The standard is not against homepages as such; it is against passing off a front page as a link to a document that lives somewhere deeper.