Applications might want to implement their own pinch-to-zoom. I can imagine that Google Maps in mobile browsers does that, otherwise you'll just see upscaled images instead of the application fetching new, higher resolution tiles.
You can't pinch to zoom the UI of native Android/iOS apps. Makes sense that web apps with "native-like look" might want to behave similarly.
It's my device, I should be allowed to pinch to zoom, etc., regardless of what the website or app wants.