See downstream <https://phabricator.wikimedia.org/T902>. Currently, timezones are rendered with their raw intenrnal 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 aleady give a pretty good error message if you do this and could improve hinting if necessary.