Im wondering about that too..cause like now if they can eat people, but rarely do that would be a different story. I wish that was made more clear.
I think that the translation is probably the one that is confusing rather than the original text (usually that is the case, not to say that mangakas don't make mistakes but they rarely do). I think that they probably said "they don't need to eat humans but the energy of the human" which means they can eat the human but it's not necessary
MAYBE THEY IT "HUMANS" BUT PREFER EATING ENERGY, INSTEAD? (￣∇￣")
chapter one page 14. Say they don't eat human but the energy of the human.
Chapter three page 42. Say they can eat human.
Did the author make a goof and forgot what was written in the earlier chapter?