Let me elaborate on proprietary Doom engines:
When id Software originally released the Doom engine, it was under a custom
license called the "Doom Source License":
http://doomwiki.org/wiki/Doom_Source_License
This license forbids any sort of commercial use, in particular, but also has
other restrictions making it non-libre. Authors of source ports like ZDoom
chose to use it instead of the GNU GPL because of a lack of copyleft in it.
Unfortunately, a huge portion of WADs are made for ZDoom and will therefore
not work in any libre Doom source ports. But oh, well. That's just something
we have to live with.
