It is not clear to me what nefarious things people believe are going on there that we should be worried about. All the article says is that people are doing things we don't know about, but implies that somehow we should feel not-okay about this. I don't know what's going on in my neighbor's house either, and it could certainly be bad stuff, but that's doesn't mean that it is bad or that I should start spying on them.
Insider trading is illegal because it undermines faith in the fairness of the public market. If insider trading is legal, then market actions by insiders are almost guaranteed to result in the action being a poor deal for the other side. Either the insider is buying shares because they have strong confidence that the price is about to skyrocket (i.e. prior to the announcement of a massive sales deal), in which case the prior holder lost out when they would have preferred to hold; or the insider is selling shares because they have strong confidence that the price is about to crater (i.e. announcing very poor results), in which case the person purchasing the share probably would have preferred not to buy at that price. The end result is that nobody wants to buy or sell shares anymore (since they will get the raw end of the deal) and the market collapses as a result. When companies are privately held, buying and selling shares without insider knowledge (i.e. deals being conditional on due diligence) is much less common, so it's really only a concern for public markets.
Public markets absolutely depend on regulators enforcing that information asymmetry is kept to the absolute minimum possible, so that the market will be perceived as fair, so that people will continue to participate.