Maybe because Imperium requires only the slightest hints to excommunicate you, making alliance with chaos followers one of your best survival options.
Plus, you know, tentacles.
Undivided is for either people who wish to "master chaos" (good luck) and reach abbadon/lorgar levels of awesome or those who try and reject it as much as possible. Or who think all branches of chaos are brilliant and don't want to offend any of the gods by excluding them (most word bearers fall into this category).
In which case, get tome of decay and sign up for the Gal Vorbak!
But yeah, ultimately nothing in the rules forces you to do anything. If you decide you want to start secretly helping the imperium, go with it. Just hope neither your former allies nor current patrons find out.
Perhaps even more than Rogue Trader, Black Crusade is the ultimate 40k RPG sandbox. Whatever you want goes, and that applies to morality, philosophy and the laws of physics as well as just your approach to specific tasks in game.
You'll note that the endless threads about female space marines in the Deathwatch forums don't pop up here. You want to be a female space marine? fine.
"But females can't accept astartes implants!"
"That guy over there has burning daemon ichor for blood and fangs growing out the side of a tail he shouldn't have in the first place and you want to argue the finer points of biochemistry?"