When you enter the page it has a simple one-field form, that promises that they wont sell or swap your details, and that you can unsubscribe anytime.
They also have counter along the left of the form informing the user how many other people have signed the petition. This will help the user feel like they are making a difference, and that they are not the only one who has signed up.
From personal experience that some charities bombard you with marketing messages once you’ve signed up to support a campaign.
Everyone hates to receive emails that they haven’t signed up for. So stating that they never sell details is also a bonus, and will reduce doubt when signing up.
When you fill in your email address the form expands to show the other fields like name, address etc. Because the user has hopefully already been convinced that they won’t get spammed with campaigns and information that they never signed up for they will be more willing to fill in the first of their detail.
It would be interesting to see how many users fill in the non-required fields. If not many, then they should remove them, as seeing a “large” number of form fields can be off putting to the user.
And if the other data collected is rarely used then i’d also get rid of those form fields too.