Why can't I change my client's email address?

You can't change your client's email address, yet you are the one who set them up originally. So what gives? 

For security purposes you're not able to change a client's email address once they have registered their account. (Before the account is registered, i.e. before the client has accepted your invitation and created a username and password, you CAN change the client's email address - say if there's a type-o, etc.)

The reason you can't change it after that? If you could, a coach could easily log in as any of his/her clients using the login helper, which would obliterate any expectation of privacy that your clients should expect (things like private comments and private Journal Entries truly are kept secure for their eyes only). 

Thus, only the client is able to update their email address, which they can do by logging in and visiting their own "My Account" page.