What happens when a user creates an email address just for a single purpose, e.g., making a single purchase, then she never uses the email address again? How s

What happens when a user creates an email address just for a single purpose, e.g., making a single purchase, then she never uses the email address again? How strong is the assumption that all users have only one email address and will keep that address long-term? I remember hearing that one could keep getting AWS free trials simply by signing up with a new email address.