Why? Because he wants to exercise a liberty.
I have an old wind-up 100 round drum I got for free.
I have no idea about manufacturer. Or anything.
The good thing about 100 round magazines is that you laugh out of sheer fun and delight at least 3 times before it runs dry.
Be sure to set up lots of targets. Especially ones like tin cans that you can shoot a dozen times, rapid fire. Set 'em on a hillside so you can keep shooting as they roll down.
What a hoot!
If someone says, "What a waste of ammo." Ignore them, they have never laughed 3 separate times during one magazine. Pity.