The reason Boyds includes a trigger guard and such with the Axis stocks is because of the design of the factory Axis stock. There is no usable factory trigger guard for this gun, thus they had to create and supply something suitable with the gun.

In terms of the rimfires, only the guns that came from the factory with the synthetic stock need a new trigger guard and assorted hardware, thus it wouldn't be cost-effective to supply it with every stock sold as half or more of the people wouldn't need said parts when replacing a factory wood stock. This is why you have to purchase them separately.