Earlier quoted context omitted.
I don't even know why this is an option that web sites can control. I should be able to zoom anything I want, site "design" be damned. It actually ruined the iPad experience for me. Early on, when you could pan and resize any part of a page, it was great; then sites suddenly decided they should "help" me scroll, choose the text size, etc. and the iPad screen ended up feeling like a penalty box.
It is important for things like HTML5 games that essentially wouldn't work without this functionality.
I understand "because games" (which are already a horrible abuse of what was initially intended as a way to display hyper-text), but far too often I wander onto some news site or blog that thinks it knows what size text I find most comfortable to read. Honestly, if you're trying to present text in the best way possible, make it easy for readers to choose both font size and column width. This lets readers adjust to suit their visual acuity and reading distance. A lot of sites seem to be designed to prevent both.