Honestly I have little knowledge about how mail to and from prison inmates is handled in Germany, I only know that the guards have the right to search letters from and to inmates and even to read them - except letters from/ to the inmate's lawyer.
What idea do I sound to be against? Searching inmates' mail or having penpalships with prisoners? I'm opposed to neither of them, the penpalships can be a good and encouraging thing.
Although reading prisoners' personal letters can be quite an invasion into their private sphere - imagine some prisoner receiving a letter from his wife with some saucy content, so prison guards must use a lot of discretion here.