They must think their sizing/font choices are good enough for everyone, so they've essentially stopped zooming.

Used properly, the META viewport should help pages adapt to smaller screens. For example, I think an iPhone-specific viewport setting would make the HN front-page more legible on iPhones by wrapping submission titles.

Unfortunately, lots of sites seem to pick settings that either don't work well on all mobile devices (like assuming iOS dimensions even for non-iOS devices) or prevent native zooming (Google mobile sites and the default WordPress mobile theme are offenders here).