Restriction on offensive spellcasting that Li Na's dreads legally circumvent, since dreads don't count.
The restriction on offensive magic is the constraint the entire Club Vanquisher Infiltration is built around. Best of all, Carl notes, Dreads don't count as offensive magic — which is why Li Na's auras can be broadcast into the club without triggering whatever enforcement would follow a fireball.
The loophole holds even as Dark Purpose causes the most destructive event in the club's lobby: guards mutilating themselves, trophy heads screaming to life, and the area formally declared defiled. The club's own alert blames demonic activity rather than illegal spellcasting.
The rule Carl's plan is engineered to sidestep.