what are these proprietary files and why are they not distributed with the program?

it should go without saying that any program that can not be built from source or otherwise will not run without some external proprietary files is not FSDG-free

From the README in the source from the git:

3. REQUIREMENTS

Abuse has the following requirements:

  SDL 1.1.6 or above.
  SDL-Mixer
  One of the following sets of datafiles:
    - Registered Abuse(original CD)
    - Abuse Shareware
    - fRaBs(Free Abuse)
  OpenGL is optional.

So it may well be one of those cases that falls under "non functional data".

From a write-up on The mentioned "FRaBs":

Abuse was a great side-scrolling action game that uses a unique system to control your character. You use the keyboard to move 
around and the mouse to aim your gun. It works pretty well and the game can be great fun to play, since it has nicely done graphics
and sound effects, as well as many non-linear levels and powerups. This version of the game is also known as "Free Abuse", which is
a fan project using the same engine (which was open-sourced by the devs) but including new levels and artwork. 

So it looks like it is just the "Graphics / sounds/ level data"

From Wikipedia on Abuse (game):

Source code release
Approximately two years after the release of the game, Crack dot Com decided to release the game's source code.[3] 
Also the shareware release's game data (excluding the sound effects) was handed into the public domain. 

So it looks like at least some of the data files are completely public domain

ok thats the key part - the "free culture" promise in the parabola social contract essentially does not distinguish "non functional data" from code as the FSDG allows - much like debian, it requires everything to be freely licensed

so presumably, "Registered Abuse(original CD)" and "Abuse Shareware" do not meet that criteria - it would probably need to use only option 3 "fRaBs(Free Abuse)", assuming everything in that is freely licensed

i.e. parabola users should not need to acquire any "data files" on their own - everything needed to run the program should be in the package