Let's rewrite this:

"The point is that public VCS doesn't benefit Wietse, and by extension Postfix, so why should he have one? A public VCS only benefits other people, most of whom aren't even contributing to Postfix development."

Like this:

"The point is that publishing source tarballs doesn't benefit Wietse, and by extension Postfix, so why should he? Publishing the source only benefits other people, most of whom aren't even contributing to Postfix development."

The answer to both question is "he doesn't have to, but if he takes seriously the goals of open source it would be silly not to." It's not unlike choosing to withhold your Makefiles, or documentation.