Priests on healing life-tapping aggressive warlocks

What’s the etiquette here? I’ve grouped with quite a few warlocks and like 80% of them will just constantly life tap during combat to the point they’re at like 10% health every mob. I’ll usually throw a lvl 1 renew on them but nothing more as it’s damn annoying and a waste of my mana. Why waste my mana instead of drinking ? Am I in the wrong here?