I assume it's because anything put in writing can and will be used against them.
DVC wouldn't be able to monitor and censor every comment put in writing and if a Guide made a comment that misrepresented the program or its rules in any way, it could open a big legal can o' worms.
Actually they can receive emails, just not supposed to respond to them but some do. When I email my current guide, she usually calls me but has not responded by email.