They don't actually ban the accounts doing this. They only remove the reviews/listings, but don't take any action on the accounts (so they just re-list 24 hours later)
Apparently when Amazon acts, they are just removing the fake reviews, sometimes the whole product but never actually banning the sellers account (even if every product that seller is listing is pumped full of fake reviews)
It seems to be a endless cycle of a item being hijacked or a item filled with fake reviews, then when reported to Amazon they simply remove the fake reviews or the product but don't take any action at all against the seller (or accounts making the fake reviews)
This thread[0] on the Amazon Seller forums is crazy with people finding products that are scam listings (with 10,000+ fake reviews), they report them, the products get taken down, then 24 hours later the same sellers have re listed with more fake reviews.
Amazon simply do not care, if they did they would:
A) Address the root problem
B) Ban the seller accounts clearly manipulating the system.
Amazon is quick to permaban accounts from real sellers, who make a single mistake (sometimes completely out of their control) but are happy to let these fake review/sellers keep their accounts.
[0] https://sellercentral.amazon.com/forums/t/review-manipulatio...