Good question. They explained it like the vultures were going to wait out the group until they die from starvation, yet they let them go look for food and don't stop/search their vehicles when they come back. They poisoned their crops, okay. They wait them out and then take their stuff, which includes food/supplies. But if they wait until they starve, there won't be any food that's left to take.
I guess they're lying to them and waiting for the right moment to move in on them. You'd think they would attack the groups that leave on runs if that were the case. Get them little by little. But with these writers, who knows?