Why do my users need a Zoho.com account?

Why do my users need a Zoho.com account?

Hi Zoho people

I am a new user of this seemingly fantastic system Zoho, and I am using it with my own domain.

I have created my users in my organization in the Control panel with username and password etc.
But when they go to zohomail.com to sign in, they first login with the username and password that I set, and then they have to "update" their account information with a new username (which is global to Zoho, so my set username (etc. "helleh") is probably already taken) and a new password.

How come is that? It's confusing for the users, why can't they just be registered in your system as "user@mydomain.com"?

Now my users username has to be something like "user_mydomain.com" or worse, something they set themselves like "iamahugegooglefan"?

Best regards,
Svend K.
teslaiqube.eu