Apple can't stop 3rd party bands. It is impossible to prevent Chinese imports.
It's impossible to stop them all, yes. If someone tries to smuggle illegal merchandise in small quantities, they have a good chance of succeeding.
If the quantities are large enough to attract notice, the risk goes up. It's not legal to import patent-violating contraband into countries that respect patent law. If customs catches the shipment, they can simply seize the items and destroy them. Whoever owns the merchandise doesn't get any compensation.
If you're buying from someone sleazy enough to ignore patents and try to import contraband, do you think they'll happily give you a refund if they can't deliver the goods because the shipment was seized by customs? The odds of a small importer getting caught are low, but the end buyer takes the risk.
If you buy from through Amazon or eBay or some other semi-reputable organization, you might eventually get your money back. But they don't want the risk either, which is why they tend to cancel the most blatant offerings of forged goods. They also probably don't want the wrath of Apple. If Apple can establish a pattern of negligent and repeated offerings of contraband on eBay (for example), they might have a strong case for reaching into eBay's pockets for big chunks of money.
None of the 3rd-party band offerings I've seen offer evan a hint that they have Apple's permission. If they were authorized, they'd very likely say so on the web page. If they were using Apple's product images with permission, they'd say so. They are
clearly encroaching on Apple's patents. If they are doing it without permission, the
only protection they have is being too small for Apple to bother with. (The alternative, challenging Apple's patents in court, is likely far too expensive for these fly-by-night outfits to consider.)