Earlier quoted context omitted.
> There are plenty of apps that are guilty of this as well. Instagram comes immediately to mind. For native apps it's a design choice. For whatever reason (good or bad) they don't want you to zoom in. For websites instead, pinch to zoom exists for a design deficency, mostly for web pages not optimized for small sized screen. Responsive web design is helping but the gap is not filled yet. Hopefully it's just a matter…
While I agree with what you are saying I have a minor nit: "For websites instead, pinch to zoom exists for a design deficency, mostly for web pages not optimized for small sized screen." I understand what you mean, but in the context of sites specifically disabling zoom it seems to me that pinch to zoom is a design feature and not a deficiency of traditional web pages. Responsive helps, but there are definitely times…
Zooming doesn't do much to give users much control over the reading experience -- iOS does not let users set or change the viewport size, so everyone gets the same columns to zoom into/out of.
If you want text bigger than the single size the designer decided on, you're forced to scroll along each line of text -- which is a terrible experience.
If you wanted it smaller, zooming out won't 'reflow' the layout, so you'll just be losing screen real estate to peripheral content.