If something is visible, there is a greater chance that a user will interact with it. Paypal wants to promote signing up, so they make the option plain as day visible to the user on their home page.
Anything that may not be as important to promote, is expected to be in a sub menu or other less obvious place, effectively leaving room for the more important items to draw the eye of users.
Assuming that if someone that is already logged in, cannot sign up (they already signed up and can't do it again), it makes sense to only show the "sign up" when a user is not logged in, and only show the logout button when a user is logged in as well.
Going back to paypal's site (or many other sites for that matter), they show both a signup button and a login button. This is because they have two camps of users, those that already have an account, and those that do not have an account.
It may not make as much sense to expect a user to press "My Account" and navigate to a sub menu if they do not have an account to call their own, compared to simply pressing the always visible "sign up" option, where they expect would allow them to create a new account.