I'm pretty confident this is to comply with a recent EU ruling that held that EU antispam laws apply to display ads that look like an email in your inbox. In essence the left prompt is a marketing email opt-in, even if it doesn't look like one (nastier people than I would say it's disguising its true nature - not I, of course).
Here's the CJEU case in question: https://www.insideprivacy.com/european-union-2/court-of-just...