How is Amazon preventing customers from signing up? How even could they?

When Amazon scrapes the seller's website for listing information and then circumvents the seller's own storefront, they're not giving the customer the information that (1) the seller has a website at all, or (2) the seller has a mailing list. This means that the customer will just never find out that information without looking for it, despite clearly being interested in the seller's work (as they're purchasing from the seller). It's Amazon inserting themselves into the process so that they can skim some money off the top at best, or extort the seller for access to their customers at worst. And all of this while the seller has created the mailing list specifically to prevent such corporate malfeasance.

What he is saying is that he doesn’t get the customers email from the sale, to which he’d start to send marketing emails.

"customers never get to interact with my website, they have no ability to sign up for my mailing list. They have no idea who I am as an artist or what I stand for," Montes-Tarazas said."

That's not what he said.

You know, what pretty much every company does when you buy something.

Pretty much every big company, yes. Small businesses are pretty careful with that sort of thing, though, because unless they want to be dependent upon Facebook or Instagram or whatever for their entire lives, they have to not make their customers upset.

“Not being exposed to me, the ‘artist’” is a perfectly valid reason, and one I would agree with. But the mailing list excuse rings hollow to me.

"customers never get to interact with my website, they have no ability to sign up for my mailing list. They have no idea who I am as an artist or what I stand for," Montes-Tarazas said."

For a solid portion of the population, if the opportunity is not placed directly in front of them to sign up for something, they will never sign up for it.