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The label of the submit button for mailchimp_lists_user_subscribe_form() is "Subscribe" which depending on the situation can be inaccurate, for example in the case of a single newsletter that one wants to unsubscribe: untick the name of the newsletter and click "Subscribe" is not intuite.
Solution: either rename the label to something more generic like "Save", or change the label with jQuery based on the state of the checkbox. Note however that there might be situation where some are checked and some others aren't, so "Subscribe" still does not make sense. Maybe "Save" or "Save subscriptions" is better.