I use PP standard and it pulls up the login prompt for the last account I accessed paypal with. I think what people are seeing is that they are testing their cart on the same computer that they access paypal from so it uses a cookie to pre-populate the email field of the login screen. It does not populate the password.
I don't see it as an issue because if anyone is going to send you money without a cart, they do it by using your email address.
Hope this helps.
Christian