These lands are usually owned by "agreement" within the "gangs" that are holding or controlling them. It happens a lot where I live until the government cracks down and everyone loses "his" land.
The reality is that they are doing temporary activity on the land (like raising cattle) and so if the government cracked down, they'll just take their cows and move on to the next land. The purchase price is too low and that makes it profitable.
At the very least one of the transactions from the article was just some random middle class guy selling land he's never owned, not some organised criminals demanding protection money.
I put "gang" because these are usually not armed gangs but rather regular dudes who have "claims" to these lands. There is a bit of power play here, otherwise the buyer will just go ahead and claim the lot.
In which case, what are they even selling? Who would buy? Even if you "buy", aren't you then still illegally occupying land?