Using Children

My son is a very naive, innocent and trusting 9 year old. Why do people see the need to pump him for information. I walk by the room and hear his grandmother asking all kinds of questions about us and our lives, my husband's family, our friends, etc. He is so trusting he just talks away. Then we hear about it later. Why do people do that. If you want to know something ask me. I'll probably tell you it's none of your business; hmm, guess that's why they ask an innocent 9 year old. Drives me crazy...Any thoughts on how to put a stop to it.