the reason is because

People say all the time "the reason is because," which bothers me as a non-native speaker because it looks so ungrammatical and yet even linguists use it in college textbooks for linguists. What is the deal with "the reason is because"? Why not "the reason is that"? I could say "I don't eat meat because it is bad for me." If, instead, I say " I don't eat meat." and then I want to give a reason, I say (or people say): "The reason is because it is bad for me." It seems like I'm using "the reason is" instead of repeating "I don't eat meat" and then adding "because" as if I were actually saying "I don't eat meat." But why?