When it comes to preventing disciplinary issues, the military much prefers simple solutions that work. This will at times result in rules which ban things that aren't really a problem, along with the things that really are, but to tailor the regulation to ban only what needs to be banned can result in rules that take two lawyers and a judge to puzzle out. Your drill sergeant hasn't got time for that.
And there are magazines that the military will see as needing to be banned. Porn (even though the on-base store sells it) was prohibited when I went through boot camp. I don't know if it was from a moralistic stance, or the simple fact that someone will steal someone else's copy of Playboy, which will probably result in a fight.
Shortly before I enlisted in the military, the recruiter held a meeting at which a former drill sergeant told us about what to expect in boot camp. He touched on the things that we were told to leave at home. I asked whether a paperback novel was allowed or not, and he said that there isn't enough spare time in six months of boot camp to finish a book. It may seem, from the outside, that disallowing a novel is an unreasonable abridgement of your rights, but the actual effect on you is minimal. Boot camp only runs from two to three months, and the last time I checked nobody ever died from a lack of entertainment. There are hills more worthy of dying on than this.
Gaming materials are probably be contraband because they might be used for gambling. That can lead to an outright brawl, and while a good fighting spirit is quite welcome, fisticuffs in the barracks are not.