It is still a big deal, because customers are going to be a LOT more hesitant to buy stuff if they realize it's sold by a random Chinese dude and not Amazon.
And, if Amazon's entire supply chain is filled with counterfeits, all the more reason why Amazon should 1) reveal who the seller really is, or 2) be liable for the counterfeits.
Given this, customers should already have been wary of purchasing from Amazon, enough that finding out that they're often purchasing from third-parties shouldn't further affect their opinion (i.e. they should already have as low an opinion of Amazon as if they had already known that, because of the other supply-chain problems.)
I've also always been a bit curious if amazon might be liable for defective products shipped through their warehouse by virtue of the transfer of custody alone. This sort of business model is quite new (it's worlds different from a flea market) and there are several interesting commercial responsibility questions around it.
What exactly am I missing here?
In both cases, Amazon loses business.
If you're selling in the US, I need to be able to effectively sue you and win in the US if you sell harmful or illegal products, full stop. So you should have to put up some kind of financial bond and a law firm to represent you. Because mom and pop businesses can't do this themselves, this should be provided as a kind of insurance policy that's just part of the cost of doing business. Heck, Amazon can be that insurance policy if they want.
But right -- if I can't sue you, you should under no circumstances be allowed to sell within the US on a US platform.
So any US corporation or LLC should meet the "US legal and financial presence" requirement that you describe.