I have no idea why Kmart is being singled out.
I think the answer to this would be A) because Kmart is more easily targeted, B) Kmart pulling the product would have an outsized impact.
Kmart is more easily targetable because they're a well-recognised brand that operates only in Australia and New Zealand, and it's a small proportion of their shops in NZ. So a public opinion campaign by an Australian organisation like GetUp has a chance of creating real pressure on the company. This wouldn't be the case for companies like Amazon or Temu that sell to many countries.
As for Kmart's impact, as a large retailer they presumably have capacity to sell quite a lot of these glasses in Australia, but I think probably more important is that a prominent retail chain in Australia selling a product has an impact in normalising that product. You can buy all sorts of stuff on the internet, for example the 'peptides' that people have been ordering lately (though who knows what they actually receive). If you see a product 'in real life' in a retail setting, though, I'd expect that to have some effect in legitimising the product in the public mind, compared to only seeing it online. That Kmart is selling a particularly cheap version of the product - under their own Anko brand - and thereby making the product more accessible could be an aggravating factor.