It may not matter to you, but it certainly matters to them. If you sign up, you become a customer.
> If I don't understand what the service/app is for, why would I want to sign up?
Try to think like the site operator. They know they don't have anything interesting to offer you, but you certainly have something interesting to offer them: your identity.
> If just signing up makes it legal for the company to email me, how does that still make me want to use your app?
Who cares? The point is to get you to sign up, reveal your personal information, not to offer you anything useful for your trouble.
> Just because I am forced to sign up does not mean I will be a paying customer or an active user.
You clearly don't understand how the Can-Spam Act works. The Can-Spam Act doesn't apply to people who are customers, i.e. people who have signed up. So by signing up, you've given permission to be spammed perpetually.