Zooming is common on mobile because sites aren't designed for it; you need to zoom, generally, you don't want to zoom around. Mobile sites are supposed to not need any form of zooming, and often disable zooming.
It seems against convention to allow zooming for mobile sites; they're supposed to be mobile optimized, and function/display more like mobile apps. You can't pinch to zoom in most mobile apps; the layout is fixed similar to these fixed viewport mobile sites.
Should mobile optimized sites allow zooming? I've heard this brought up as an accessibility/control issue, but accessibility options are generally set up in your browser--make the font larger ect. Pinch zooming is a hassle and on mobile layouts it doesn't make very much sense. But when should it be allowed? What benefits does it give the users or designers?