the US has had lots of programs where labor can effectively be bought or contracted from prisoner sites by private companies.

I know of prison ran machine shops that were doing die-casting and tool production. I also heard of one (didn't see) that was doing basket weaving for a floral/arrangement company.

these are shallow 'social benefits'; but the companies were privately owned.

I guess the classic example is license plate pressing.. I guess that's a social good? I don't know if it goes on at all anymore.

Why do they have to stay inside? Have a chain gang trim overgrown weeds along roads, fill in potholes, clean leaves, clean and repair sidewalks, plant shrubs, etc.