I feel like this is probably more the reason than any deliberate effort by Unsplash photographers to troll Medium users. Because the image database is entirely royalty free use, it's likely photos are accepted from some who wouldn't make the cut if they were paid for their images. I find similar issues on Pixabay (who also offers vector files) and Pexels – though not as often nor as pronounced; perhaps that's because those sites also offer a full-scale print-reproducable copy for a royalty fee.

As an artist, art photographer, and graphic designer myself, I full agree with your take on the display of images. I have used taller images in the past – but that was when you could left justify the images and have the text wrap to the right of the image. It was actually a favorite mode of mine for poetry. As you know, displaying poetry is often akin to an art installation.