That technique should not be hard for the agencies to defeat. They simply need to encode the image url instead of using it in plain text.
And note that this video is from a year ago. Yet one of the commenters said it no longer works as of July 2017 -- which implies that 500px saw they were being hacked like this and made a fix.
Yes, it is disgusting that people steal our photographs.
Now... do you steal songs or movies or TV or software or ...? It usually seems that people are only upset if their specific material is being stolen, yet come up with excuse after excuse about why it is OK to steal someone else's material...

And for the record, I pay for all my music (mostly from iTunes), all my audio books (almost all from Audible), all my movies (either iTunes or Amazon Prime), and all my software. If I feel someone should not steal my stuff, then I don't feel I should steal anyone else's stuff either...