Absolutely nothing is stopping them, and each company has a specific program to ensure that their medicines get on the market at a reduced price, meaning a price that is different from the price in the north. Also, these companies have the exact same systems in place to make sure that the drugs will not go back north: here they have a different name, a different colour and the labels on the packaging are different.
The companies are already part of a dual system where the north pays more than the south. But the difference between the price of patented drugs and the price of generic drugs is so big that this is not an option for most markets where the drugs are needed.