We will add mention in the example text for instamail that the email will include a promotion of our service.
If enough people are interested we could add the ability to remove the promotional text at a cost of 25 cents per instamail. We have few ways to attract new customers and that's one of them, so we cant just take it out. If we take it out we need to get new customers elsewhere which means expensive, mostly ineffective advertising which is expensive. We monitor all signups and how they came to signup.