The fact that it was legally mandated is good evidence that it is [necessary].

P1 If X is legally mandated then that is good evidence that X is necessary.

P2 slavery is legally mandated (in the USA through prison labour)

C1 There is good evidence slavery is necessary.

P3 [insert other silly/abhorrent thing here is legally mandated: ICE in general, state murder, HOAs…]

C2 There is good evidence silly/abhorrent thing is necessary.