If I loan you mine
I'm happy to let most folks shoot any of my guns at the range. I will also supply the rounds unless of course I know with certainty that what goes in my gun is factory ammo.
I reload, I know what I put together and have no qualms shooting my ammo. Your ammo? Sure in your gun, not in mine.
The range that rents guns is no different. They have considerable dollars tied up in their inventory, it's fair for them to protect their investment.
If the prices of "their" ammunition are outrageous? I shall exercise my rights to spend my dollars elsewhere.
I do fully understand and agree with the policy of factory only in rental guns.
I also know of ranges where only "their" ammo is permitted. In that case it's most always overpriced, and again I exercise my right to spend my dollars elsewhere.
I can't imagine a skeet or trap club that prohibits reloads.