Why does WP recommend against custom favicon functionality in themes?

I am developing a WordPress theme, and when reading the Codex article on Theme review, WP recommendeds against allowing custom favicons in a theme. Does anyone know the reason for this recommendation?

From the Codex:

Favicons
Themes are recommended not to implement custom favicon functionality.
If implemented, favicon functionality is required to be opt-in, and disabled by default.
If implemented, favicon functionality is required to support user-defined favicon images

Probably because a thing like a favicon is not necessarily tied to the on-page design, but to the branding of a site in general.

In other words: it’s not specifically part of the “display” of the site. If a theme does choose to implemented it, an ender user might not want it (opt-in only) or they may want to use their own favicon (user-defined) to fit with their branding.