I would say they are homemade, or maybe shop-made, not a commercial item. On the ice-cream type bikes, you need some weight for stability. Husky sells their 3-wheeler, and recommends a minimum of 150 lbs in the box portion to ride. Worksman's 3-wheeler probably weighs 100 lbs. So using plumbing pipe is not a bad thing from the weight standpoint.
One thing I'd be curious about is that the regular Ashtabula cranks aren't all that wide, and the tires are. So when you put a 2.125" wide tire between those two pipes, it's likely to be a tight fit between the rotating ends of the cranks.
One of the oddball items on Worksman trike is that front yoke, which is not a normal bicycle or automotive part. I'd be curious how they worked that on these, as well.
__________________
"be careful this rando stuff is addictive and dan's the 'pusher'."