Because the kid was obviously a freak. Between the way he talked and acted, his father didn't want his son reading that stuff. Look at the kid's reaction to seeing a skeleton at his window. Look at him in the end with the doll. The kid was a freak.
shareHe seemed to me like one of those fathers who wanted their child to stop focusing on imaginative creatures that don't exist and get in the "real world." I have a feeling Billy's "weird" personality caused him to be kind of a loner. When I was younger I didn't meet anyone who enjoyed old horror movies like me until 4th grade. They don't seem like a very common interest (at least not where I live). Maybe his dad reasoned that these bizarre comics were what fueled his bizarre personality, so taking them away would make him normal. Maybe he just wanted his son to join the baseball team and make friends instead of sitting alone in his room having an "unhealthy" obsession with monsters all day. I don't really agree with that but I think that's probably close to how he felt.
Death lives in the Vault of Horror!