I want to be clear: are you objecting to the notion that good characters, on balance, tend to cooperate more easily than do evil characters?
I'm not sure that they do. Take the stereotypical Mafia Family scenario. The group operates and cooperates for years without any real problems, so long as everyone stays in their place. In many Mafia movies and TV shows, it's because of one (or possibly a small, splinter group) member of the group that problems occur.
D&D does not engage in complex moral relativism. The system doesn't acknowledge it, and the mechanics don't support it. So while you're technically correct (the best kind of correct!) it doesn't really have a bearing on this discussion. Good characters tend to cooperate more easily, and D&D works best when the PCs cooperate well.
Having run and played in more than a few straight up evil campaigns recently, I've found that evil groups function better than good ones to be honest. Good characters (and players of good characters) don't feel the need to cooperate because they know that if they don't cooperate, the worst that will happen to them is they will get yelled at by the other players.
When evil characters step out of line, their player's tend to have to roll up new characters. So, they cooperate. When conflicts between characters do arise, they get resolved very (sometimes
very) politely. Because everyone at the table knows that this can escalate to a degree that a good group never will.
I know that this is a tangent to the OP, but, it's an interesting one. I would hope that if they do put an "Evil Campaign" section in the DMG, it will include a lengthy discussion about what steps the DM and the group can take to make sure that the whole "I steal from the party 'cos I'm
eviiiiil" thing gets nipped in the bud.