There has been a lot of discussion here, but let me try and throw a little water on the flames...
There is a huge range of business models out there. When it comes to warranty service, much depends on the size of the gun company, the size of the distributor, and the volume of trade.
The Marstar's of the world may deal with warranty issues somewhat independently of the manufacturer. They'd bring in some spare parts and price the product to cover their loss if they need to simply replace a defective gun.
The R.Nichols of the world may have a specific set of guidelines from the manufacturer about what they can deal with locally, and what must be approved by the manufacturer.
In the middle, you have the murky grey areas.
If the distributor doesn't have spare parts on hand, they look bad.
If they keep a full stock of spares, and sell them at retail when folks bend/fold/mutilate a part, the customer may be happy.
That same customer might also become indignant if asked to pay for a part they don't feel they abused.
If the distributor has some discretion, they can keep the customer happy, but risk being stuck with the bill by the manufacturer.
On the balance of things, I'd rather see the distributor carrying the parts in quantity.