[MysticCat]

Basically,

(1) such a group can be considered an arm of the chapter, so that a fraternity's liability could extend not only the acts or negligence of a chapter itself but also to the acts or negligence of the auxilliary group;

and

(2) the functioning of a fraternity chapter and an auxilliary group as a common unit at social and other events could jeopardize a fraternity's single-sex status.