Eliza trembled in rage… Zane and the girls had just dealt a significant blow to her already limited power. At this rate, Bart was likely to march into her territory and seize control of her horde. Most likely devouring her in the process.
Without sufficient power to deter a neighboring Horde King, especially one as close as Bart was to Eliza, then a Horde King would at best be subordinate to their neighbor, and at worse, be destroyed. Such was the nature of their caste.
Even if there were rules in place to prevent this, especially currently while they were at war with the living. The fact remained that the only Horde Kings who would enforce these "rules" were those who sat much higher in the hierarchy.
And there were any number of excuses that Bart could make to Lilith, who was effectively in charge of what had previously been the Western United States, to get her off his back if he were to swallow up Eliza's horde.