I can't believe that I just paid for a 6 month subscription to LWN, only to read this.
This article is massively skewed as it implies that Okular tries to force DRM onto the user.
The truth, as explained in a comment above, is that DRM can be disabled most easily in 4 clicks at the most obvious place in Okular's configuration dialog.
Moreover, it is up to the linux distros and/or sysadmins to define the default configuration. By making this a regular KDE configuration entry, okular makes it most convenient for the distros and sysadmins to decide.
Thus the whole discussion about "should be opt-in" is moot. At the very worst, Okular can waste the user's time doing 4 obvious clicks. Doesn't the author have anything more important to write about?
It is unfortunate that the article failed to mention this very important fact and thus gives an impression of Okular that just doesn't match reality.