Users login and customers tables

The shopping cart I am using has the option to create customer accounts that are integrated with WordPress users. So there is a table for users and a table for customers that are tied together with ID. What is to stop someone who is familiar with WordPress to find my login page, enter user name and password and have access to the backend? I see it records in the meta data that they have “subscriber” status. Would they even be able to log in to WP with that?