This may be intended but I expected health not to regenerate even in Peaceful mode after having set naturalRegeneration to false.

Steps to reproduce :

• Set gamemode to Survival
• Set difficulty to Peaceful
• Enter the following command : "/gamerule naturalRegeneration false"
• Make sure your hunger bar is full
• Take damage
• Notice that your health regenerates

The problem is that a survival player in a world where naturalRegeneration is "false" could easily heal by setting its difficulty to peaceful.
So I consider this is a bug.