Why the asymmetry? And what do you have in mind when you refer to "two-step sign up"?

For my convenience; and by two-step sign up I mean "enter my e-mail into a form, then receive a confirmation e-mail and click on a confirmation link to be actually subscribed".

Are you confusing this with email newsletter subscriptions? This is talking about paid membership services, the kind of thing where you need an account and often payment methods.

Yes :) Sorry.

It would stop my email doppelgänger from signing up for services under my address.

Sure, but it's a minimum. Also, the incentive here is aligned with what's best for the customer, i.e. they probably want to make their two step subscription process to become one step which automatically gives the user an easier process to unsubscribe.