Why are people so mean to my son?

My son is 8 and a half months old. He is a very happy baby. He sucks his thumb a LOT...when he's happy, when he's sleeping, when he's just sitting in your lap. Lately I have noticed that certain people insist on pulling his thumb out of his mouth. He either puts it right back or starts crying. Why do people do this? It is cruel!! No one ever pulled my daughter's pacifier out of her mouth!