While the Contact Lists API does allow you to add and remove contacts from static lists, you won't be able to send to contacts in those lists unless they are opted into the subscription type of the email you are sending. The Email API is used to control those subscription statuses.
For example: Jane Doe is currently opted in to your "Marketing Information" subscription type and is a member of the "Marketing Mailing" contact list. Jane goes to your site and opts out of email communications related to the "Marketing Information" subscription type. If the module uses the Email API correctly, Jane will be opted out of the "Marketing Information" subscription type. If you send an email with the type "Marketing Information" to the "Marketing Mailing" list the next day, Jane will be excluded because she has opted out of that subscription type — even though she is still technically on the "Marketing Mailing" list. To opt back into receiving "Marketing Information" emails, Jane would have to either access a previously sent email's footer or fill out another one of your forms (with resubscription emails enabled) and check the box confirming she wanted to receive a resubscription email.