Well, in all fairness, Doom 2 was a mission pack with one new gun, it wasn't much of a sequel as they didn't really change anything from the first game.
In the old days, that kind of thing was allowed.
Today, the only games that are allowed to do that are sports games. No other games are allowed to have sequels that are basically the same game with on the same engine with same graphics. It just shows the developers are lazy.
Also back in the day, there is the shareware thing.
You know about wolfenstein 3d right? When it was released, from stores, you could only buy episode 1 for cheap price, and get the rest through mail order direct from id. The commercial release would be spear of destiny.
Now for doom, before ultimate doom, which came out after doom 2, the original doom release in 1993, you were only allowed the first episode for free, or for a small price of shipping. You can't get episodes 2 and 3 from stores, and you have to order direct from id. Doom would be the registered version.
Doom 2 is the commercial version of doom 1, and seeing how they don't want to rip off the people who bought the original doom, they made more levels and more enemies, and the one new weapon.