Slimes and MagmaCube don't drown

Hello! I was in my 14w28b server, when my friend and I discovered that slimes and magmacubes don't drown. We were just playing around and we saw that the slimes and magmacubes don't drown while in water. We tried spawning them on the ground level of the ocean and they actually can float in water without drowning. I have no idea of this is a bug, or normal