Why are they illegal in Korea?
They're not. Or at least, authorities don't seem to care.
I meant: They're illegal in the absolute because they're just Brompton knock-offs, and the bikes would be seized if they dared selling them in the West. Which is why they don't ship overseas, especially to Western countries.
As to why Korea allows those bikes to be sold there is a mystery. It's a developped nation now and part of the WTO, but it looks like local judges don't care much about intellectual property when the owners are foreign companies.
Incidently, look what happened to Chedech, also from Korea:
https://www.bike-eu.com/laws-regulat...-case-10138237
"The CJEU ruling in Brompton’s case is potentially significant for many such creators, as it confirms that the prevalence of creativity in their designs may afford them copyright protection. However, it’s important to reiterate that the Brompton case is not yet over. It will be for the Belgian court to decide whether the design of the bicycle displays creativity and originality. Other recent cases have shown that the outcome of court rulings on copyright can be difficult to predict."
https://www.farrer.co.uk/news-and-insights/brompton-puts-the-brakes-on-copycats-but-its-not-over-yet/