I don't think Joyce was a loon, and I don't think she was being too hard. I do believe that she was bluffing, thinking that Buffy, even if she did "go out that door," would be coming back. I also believe that she was being a protective mother who had just suddenly had a load of to say the least extremely disturbing news dumped on her. "I'm sorry, Mom, but I have to go out and kill a bunch of things that you only thought existed in horror movies, and, by the way, my boyfriend, the one I slept with at the age of sixteen? Well, he's a four hundred year old vampire who's planning on destroying the world, and I have to go and try to stop him. I may not survive the night, but if I do, could we please sit down in the morning over toast and juice and discuss this then? Oh, and just in case you're worried about me getting hurt, you don't have to, really, because I've already been killed once. It's no big thing. Now, if you'll excuse me, I've gotta go and save the world...again." Nope, I think Joyce showed remarkable restraint in not trying to tie her daughter up and call the guys at the funny farm to see if they do night pick up.