It does force that the person has a certain custom capability, if you protect that page.
You can use the
success="" attribute if you want the button/pro-form to re-direct to a specific page after registration/payment is completed.
You can find out more about the
success="" attribute under
WP Admin -> s2Member -> PayPal® Buttons -> Shortcode Attributes ( Explained ).
Or if you're using pro-forms you can find about more under
WP Admin -> s2Member -> PayPal® Pro Forms -> Shortcode Attributes ( Explained ).
Hope this helps.