Could someone explain to me - for the seller what's the downside? Isn't this just more sales? Isn't that positive?
Losing brand control (IP) and lack of control over your sales channels is a big problem for traders who want to comply with dozens of regulations, exclusions and restrictions required by manufacturers, other sellers, trademark owners etc. The article also mentions means tax and customs compliance problems, but this also affects other trade law related issues, like competition law problems (antitrust). It is surprisingly easy to breach these even for tiny sellers. You don't want exposure to these kinds of sales. It is never just selling, it is always also complying with lots of new laws, And rest assured, Amazon bots will not work that out for you. And now, these guys have to scrutinize the email orders as for their source because all the compliance process they have built in their webshops or wherever they advertised, will simply be bypassed by Amazon bots. Terrible idea.
Small brands put a lot of energy into their sales experience and aftersales support. They are trying to get repeat customers.
Amazon leads to one-off sales, with your product becoming indistinguishable from hundreds of different stores. There is no reason to return to your store, because the customer isn't even aware they bought from your store!
And it can get nasty real quick if you are the only seller and Amazon messes up. A negative review of your product due to bad handling by Amazon means you are now being held responsible, in the eyes of the community. A widely-spread "I bought a $500, it was packaged poorly arrived broken, and they refused a replacement/refund" can easily kill a small company - especially if they try to do aftersales and conclude that the complainer never ordered from them - who then of course shows pictures seemingly proving the opposite...