It's a way for them to make sure all users understand Google respect the users' email privacy. It's pretty much a no-brainer for Google: It's extremely cheap way of hammering the message, and they are making sure that no competitors have an angle on getting users from Gmail on grounds of privacy.
"You should use us instead, we would never disrespect your privacy by reading your emails!" - well, nobody can use that argument, because Google doesn't either.