I agree with you. If they don't have the time, or have very limited time, they should say so. But another thing that might give some a bad attitude is that some people aren't grateful or courteous enough to thank them for their help. I am a volunteer myself, putting in quite a lot of time looking up stuff in books for people, which I certainly don't mind doing, but I like to know it's appreciated. Maybe they just forget to get back to me. But, all things considered, I won't mistreat anybody or make them feel stupid about queries they send me.