in the WA_eCart/PP_ECO_Scripts/PP_ECO_PHP.php file, the solution type and landing page are set to Sole / Billing which are the settings according to the paypal documentation that are needed to offer Paypal Account optional:
"<SolutionType>Sole</SolutionType>".
"<LandingPage>Billing</LandingPage>";
https://developer.paypal.com/docs/classic/api/merchant/SetExpressCheckout_API_Operation_NVP/
The code looks to be set properly to not force a paypal account.
There must be a configuration setting in your paypal account overriding this.