Why are some websites not zoomable on iPad?

Does anybody have any idea why iPad zoom (you know, where you use two 开发者_StackOverflow中文版fingers to enlarge the text/screen) does not work on some sites?

For example:

• facebook.com
• c3.arc.nasa.gov/nex/ ± the site that I'm working on.

Google search does not reveal anything, which makes me think it’s not a common problem.

Thanks beforehand for any insights.

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This lack of zooming is by design; the website has chosen to disallow zooming. It's done with the viewport meta tag. For instance,

<meta name="viewport" content="initial-scale=1, maximum-scale=1">

prevents zooming.