The data captured on behalf of our merchants belongs to those merchants, it’s as simple as that.

No, it doesn't. It's the end users data.

and this isn’t possible when Mailchimp locks in their data

Complaints about lock-in from you is pretty rich.

Mailchimp refuses to synchronize customer information captured on merchants’ online stores and email opt-out preferences.

You're intentionally misleading the reader here. This isn't about opt out preferences. That's just a useful excuse.

Any idiot can see you're trying to build up a complete picture about user's spanning across stores.

This isn't about your customers, this is about you wanting to push mailchimp out.