It's not just two adults, it's still between a father and son. Although corporal punishment is commonly linked with children, it can still happen between adults.
A good look into this can be found here and here.
Basically, corporeal punishment is something that is controversial in the U.S. A possible reason why this was changed was to avoid having Ryoma seem like an abusive father.
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Hunter Nightblood
Mar 02, 2016 at 11:58AM EST in reply to
Hunter Nightblood
Mar 01, 2016 at 11:33PM EST