Many reasons. A popular one would be to flip through a gallery of photos related to the story. That one's VERY common.
Also, I missed mentioning the [X] button on the AMP header banner. I bet that most end users think that [X] button should make the AMP banner disappear, while leaving the page content there. Pretty common pattern, like for the EU cookie warning. That's not what it does, though.
Back to the "right swipe", at the very least, it should do nothing, assuming that an AMP page belongs to the publisher.
Ok, sure. Going through a slide-show makes total sense. So, you're saying that there's no normal behavior on a plain website for swiping right (yes? I don't use mobile much myself), but AMP adds a Google-centric swipe-right function regardless of whether there would otherwise be no swipe-right function or some other expected one. Am I understanding now?
Yes, that's it. Google hijacks the back button, swipes, and the [X] button on the AMP banner...in favor of Google, and to the detriment of the publisher.
That is indeed horrible. I am glad to learn that there isn't otherwise some new trend of people constantly right-swiping for no apparent reason though.
It certainly makes it clear that the content owner of an AMP page is not the page owner.