As Kyle Schaeffer put it:
You should only disable the zoom feature if it enhances users’ ability
to consume content on your site. If you’ve formatted your design
layout so that users don’t need to pan or zoom, the zoom feature
actually impairs the user from navigating your content (which only
needs to scroll vertically). If you’ve incorrectly formatted your
layout, however, disabling the zoom feature will only further
infuriate your users, as you are removing a very important navigation
capability of the browser.
An article on HTML5Rocks says about a particular implementation:
In order to accommodate for sites not optimized for mobile screens,
many modern mobile browsers set a larger browser viewport, which
allows for better viewing of non-mobile-optimized sites. Users can
then pinch-to-zoom in on the content they want. That's fine for
non-mobile experiences, but because we're optimizing our experience
for mobile browsers, we'll use the viewport meta tag to set the screen
width to the device width.
It's important to note that we're not disabling the user's ability to
zoom the page (which you could do by adding user-scalable=no to the
content attribute), even though we're optimizing the content for small
screens. It's recommended to keep user zooming enabled to keep things
as accessible as possible. However, there are use cases to disable
user-zooming, such as if you're including fixed positioned elements
And a quote from Luke Wroblewski is relevant here too:
Just because my screen is small doesn’t give anyone insight to my
behaviours, desires and needs.
i.e. why should you restrict the user if you don't need to. Let the user decide!
It's not impossible for users to zoom within a responsive web design, in fact it's very easy to allow it (or rather not disallow it) - it's just the resulting appearance and interaction can seem awkward at times, depending on how well/badly the content is structured.
One option is for users to pinch zoom until the next media query hits - such as described in this REMux proof of concept on CSS Tricks.