No, I don't think its up to the guide. I think Disney tells the guides not to use e-mail so that they have more control over the written correspondence. All the documentation they send in writing is stuff like official forms. Everything shown in person is that way as well. If they were able to e-mail there would be greater potential for misinformation to get out and it would then be documented. I think the legal folks at Disney are just covering their butts (my CYA comment above) and don't let them "just in case."
I'm saying the guide who said they weren't allowed because they do real estate is using that as an excuse, but it's not the real/entire reason. Not saying they are being sneaky or anything. They just don't want to bring up the L word (lawyer). Many resale agents, who also therefore deal in real estate, do use e-mail on a regular basis, so there's no state law or anything prohibiting them from using it.