“The selected images are download and added to the WordPress media library, saving users the trouble of having to leave their dashboards to search, download, and upload images.”

https://unsplash.com/license
“Photos cannot be sold or redistributed without significant modification.”

If the images are downloaded and put in the media library for you to use (in posts for example), then you’re basically redistributing them.

Also, the license as a whole is against the GPL.

Promoting this plugin and hosting it on wp.org shouldn’t have happened, even if it’s just a service gateway.