They can monetize by presenting ads, and they can identify you with cookies when you don't log in (or later when you do). They don't need to restrict their content to monetize. It's more likely that they are restricting their content to prevent scraping and/or indexing.
It's definitely to prevent scraping. Used to be you could scrape and hydrate any profile or hashtag. It was sort of a cat and mouse thing a couple years ago and now the Instagram website is locked down tight.
