I mean, come on. Everyone uses email. Having said that, everyone also knows there are things you put in email and things you only discuss face to face, or over the phone.
And the dividing line doesn't necessarily have anything to do about breaking the law.
Everyone on wall street has had an email they typed up get into the hands of someone who they wish it wouldn't.
Whether its shitting all over an analyst for not being good at their job that got back to the analyst or doing something similar to a CEO, these things can harm your future relationship with these people who you may need to do business with again.
You rant over the phone, you issue actionable orders via email for the same reason. One has no trace, the other has a record you can point to.