Yeah it is a web browser, and one of the oldest job of a web browser is to turn image files and markup into a visual representation. And if you want to somehow capture this visual representation as an image, the best way to do it is through the browser, since the broswer is the authority on how the pages are being rendered.
Previously you had to download an add-on to do this, which is a bit of a pain, and something the browser should support natively, if for no other reason than it is trivially easy for the browser to simply provide the data it already has.
No the browser shouldn't do everything. It should only do things that are browser related. This is browser related.
The article sure seems to describe what simply is a cropping-capable screen capture tool tied to a cloud-based, social web back-end. I believe this is another damn data grab and one disguised by a so-called needed feature. Even if not a grab per se, it is one more way to get the user-base locked into cloud-dependent services and applications. I'm certain that the mainstream user doesn't need or want this and that the user isn't taking and indexing screen grabs. If this was capable of dumping the entire page as rendered to something like a single PDF or image file, it might be useful since, as you've suggested, the browser would be best to do this. It seems to not do this, so why even add the feature?
Why add the PageShot feature to the browser when the OS already allows the user to easily capture what's on the screen and has an image editor? A side from the OS, countless add-on apps are available for installing that will grabbing screen captures all day long through the click-drag-and-highlight model that will do this regardless of what is running or displayed? While I am not a big fan of the extension model mostly because people attempt to make the browser do too much, the browser's core functionality should be supplemented this way to keep the application's file space and resource footprint as small as possible. It also keeps the browser's dev team focused on the core functionality. Let third parties cater to niches.
I have gotten by just fine in Windows with PRNT-SCRN and ALT-PRINT-SCRN. For other OSs, its just another key combo.