It's a rather common practice on instagram. In the absence of some sort of official "retweet" mechanic, this is what people have taken to doing. As he mentions

It's a rather common practice on instagram. In the absence of some sort of official "retweet" mechanic, this is what people have taken to doing. As he mentions in the article, the response is often very positive from the original content creator, since he credits them (and presumably drives traffic to their property).