I believe this is exactly correct. Email is a 'paper trail' and being able to change that paper trail ex-post facto benefits the sender waaaaaaaaaaay more than it does the receiver. I met an engineer from Google who quit when they insisted on "dogfooding" this.

They used the example, you send an email that says lets meet for dinner tonight at 6. You arrive and after 30 minutes begin to wonder, go back to your email and now it says meet "tommorow night" at 6. Are you crazy? Did you misremember? Or did the sender change the email after they sent it and you read it? How could you complain?

As I understand it, it was met internally with "that isn't what we mean." But the ability to send HR important announcements and then change them after the fact is a capability that is just too tempting for HR to resist at some point.