Not a bug. It's because people wanted Pyro+ to make them immune to magma blocks. (Magma blocks aren't fire damage though, they're the same damage as cacti. They're nether cacti.)
Are you sure? I can't find any reference on this, on the contrary: it seems magma blocks do deal fire damage (unless I'm missing something) http://minecraft.gamepedia.com/Damage#Magma_block
What I understand from this is that cactus contact and walking on magma should actually be treated differently according to bukkit/spigot, as there's specifically two distinct values: CONTACT and HOT_FLOOR
"Magma blocks damage living entities standing on top of them" source: http://minecraft.gamepedia.com/Magma_Block#Damage
