Why can't guests save photos when they open the link in Instagram?
Instagram's and Facebook's own browsers can't save photos to a phone, so the gallery shows guests how to open it in Safari or Chrome.
Opened inside Instagram, Facebook or a similar app, the gallery shows a dark line at the top saying its browser can't save photos to the phone. Then:
On an iPhone, Open in Safari opens the gallery there in one tap; on Android, Open in Chrome.
Or Copy the link copies it, to paste into the phone's browser.
Or they tap ··· at the top of the app (⋮ on Android) and choose “Open in browser”.