> the marketing team is honestly stupid if they don't do this. If a customer has purchased something, companies do everything they can to encourage a repeat purchase. Part of that is offering newsletters on updates for new products or sales.

Everyone unchecks those boxes anyway. The ones that don't and actually want that crap are engaged enough in the product/company that they'd gladly sign up for the spam if you asked right on the front page or anywhere else but the checkout screen. Why annoy the crap out of 99% of your customers so that you can catch that 1%?