Guys, think I'm not being clear... my point is that because the form is incapable of knowing if the lead viewing it has opted in already, you have to default an Unsubscribed-linked checkbox to unchecked (on this we seem to agree).
And even though some leads might have turned out to have already opted in, it doesn't make sense for an unchecked box to have no meaning for previously opted-in leads, while being interpreted as Unsubscribed=true for new or previously opted-out leads. I feel it should have the same semantic meaning in all cases.
The problem is solved by never displaying a checkbox linked to the Unsubscribed field unless you have made a good-faith effort to Pre-fill it. In this case the form is not ready for prime time IMO!
Thanks for all the feedback guys. We're on board with not checking the box by default. We just need to make sure that if someone is already opted in and then fill out an additional form (leaving the box unchecked) that they don't get opted out, which is what is happening today.
"My fix for this was to create a new custom field specifically used for the form, and then a triggered campaign that listens for what the person does on the form. So if the box to register was unchecked, then I ignore. If the box to register was checked, then the smart campaign runs to change our actual subscribe field to true."
You can use a proxy field as Dory describes. If you don't do this then unchecked on the form naturally means Unsubscribed==true.
I know I'm a broken record, but I still don't like this design. You're showing someone a checkbox that seemingly has a specific meaning but that you'll willfully ignore.
I suspect what you may want to do is create a separate field titled Opt In date. You can then send all marketing campaigns to people where this field is not empty. You'll need a separate smart campaign to auto fill in the opt in date box upon successful form submission where the checkbox is filled in.