I suspect it is to force the inmate into having to purchase all items through the facility's Canteen that has arranged a contracted monopoly to provide convienient store sized products and selection at resort town prices. Books and magazines usually must be sent direct from a publisher or commercial business. Even newspaper clippings can be refused if the facility considers it harmful to the security of the facility like a story of a riot in another prison or recent Supreme Court rulings. Sometimes printouts of internet web sites and files from the internet are refused. If you do choose to send money to an inmate check with him first.
Every state has their own rules and often in complete oposite of other states.
Some allow money from other than direct family, others don't have any restrictions and some do not allow any money from other than family. Some insist on postal money orders and some only personal checks. I gave up keeping it straight for fifty states plus federal facilities. As always, in all cases and subjects, when in doubt write the inmate first to get the skinny on his particular facility. If you are concerned about this then perhaps you should not do this.