Historically, hotlinking images is considered rude, at the very least. But there is a service called unsplash.com that has a business model around hotlinking of images.
Unlike most APIs, we require the image URLs returned by the API to be directly used or embedded in your applications (generally referred to as hotlinking).
Source: https://unsplash.com/documentation#hotlinking
This means that image proxies, like those routinely used by steemit.com, are in violation of their terms of service if the image was provided by the Unsplash API.
In practice, this means that general users who just link to an Unsplash image that is then proxied by the steemit.com image proxy is not in violation of the license. But if a developer writes automation that utilizes the Unsplash API to create a post that links to an image, it can result in an unlicensed content on the frontends.
Thus, Unsplash may be within their rights to selectively request a DMCA takedown for certain posts here if they have reason to believe their API was involved.
The spirit of the license is that they want to collect data on image usage. In doing so, they do not want to provide their competitors with free image hosting.
I don't really think they'll ever go after Steem-based front-ends unless that front-end is trying to implement the Unsplash business model. But it's something to be aware of.