See downstream https://phabricator.wikimedia.org/T902. Currently, timezones are rendered with their raw internal names (like America/Los_Angeles) which include underscores.
Replacing underscores with spaces is a more human-readable (and perhaps meaningfully better for things like screen readers, although this is pure speculation).
There's some vague argument against this, like "administrators may need to set a raw internal value in phabricator.timezone and this could mislead them", but we already give a pretty good error message if you do this and could improve hinting if necessary.