Not at all familiar with patent law, I take it?
IP is more than just patents. Brompton tried to sue Dahon when they introduced their Curl Brompton clone in Europe based on IP (and not patents) and lost the case, as far as I know because the court came to the conclusion that the Curl was different enough from the original. As none of the other Asian clones are offered outside Asia there might be reasons for that. One being that those plagiatory bikes do not fulfill western safety standards and have not been successfully or at all tested to the norms mandatory for selling in western countries. Second may be that in the early 2000s companies like Merc that did try to distribute their Brompton clones in Europe got sued by courts due to violation of IP (and obviously not because of patents as those were already gone by then). The bikes had to be destroyed and they were explicitely banned from distributing those bikes in Europe.
Even in China, where IP infringement of western brands is notorious and seemed impossible to stop things may be about to change: Land Rover just won a case against Landwind, selling a clone of one of their car designs:
https://www.dailymail.co.uk/money/ca...er-Evoque.html
With that happening climate may be changing for copycats in China. Brompton is trying to be more present and sell more in Asian markets and thus they may sooner or later have to address the issue locally - which they did not do until know, possibly because of risk, limited ressources and only a small chance of winning the case due to the general attitude in China towards such things and even less of being able to enforce a possible outcome towards the ever changing amount of clone producers. Instead they focussed on developing their bike further to beat the competition qualitywise. The future may however show a different approach.
Just because there are no patents involved or Brompton did not go to court does not mean that what the plagiators do would be legal.