[smacktoward]

They want you to use email because they want to get your email address and opt you in to their email list. (Note that you opt-in by default unless you specifically choose to opt out.) The messages that go to the friends are a nice side benefit, but they're not really the point of the exercise.

A valid, working email address is very valuable to fundraisers, because they can then hit you up directly in future for additional donations. Can't do that if you tell people to go offline and call their friends.

This also answers the question elsewhere in this thread of "why not ask people to share it via Facebook/Twitter instead of email?" In those cases there's an intermediary (Facebook/Twitter) who can stop your marketing messages. So your email address is more valuable to them than, say, your Twitter handle.