The intent of that clause would have been to allow them to a) actually show the images on the 500px site, b) highlight them in "popular images" on the site, c) surface them in API results.
Agreed. Plus the terms do state that all copyright remains with the user. Someone seems to have amended the basically text to reflect this:
Your photos will preserve whatever copyright they had before
uploading to this site. We will protect the copyright and
will not sell your photos without your permission.
