That's the way it was written.

You said you're using Zen Cart v1.5.1 and PayPal Express Checkout. The way Zen Cart v1.5.1 works with the "Automatic Account Creation" in Express Checkout may seem confusing. Zen Cart will always create an account for the customer who uses Express Checkout. The "Automatically Create An Account" setting only controls whether that account is treated as a permanent user-account. If "Automatic Account Creation" is ON, then your Zen Cart will send a welcome email to that customer, along with a generated password for that account, and they will be able to log into that account for reviewing their purchases or to make new purchases. If the settings is OFF, then that welcome email is not sent, and the account is flagged as internal-only and no password is issued.
The account is necessary for the internal workings of the store, which is why a record is always created in the customers table regardless of whether "Automatic Account Creation" is turned on or off.