It's the most blatant example of insider trading. Company insiders might have inside access to a single company. Lawmakers have access to inside information that can affect the entire industry or market. It's a perfect example of how we do have class stratification in society and that rule of law and equality are laughably distant ideas in a sea of hypocrisy.
Remember that ONLY elected congresspeople are exempt from rules like that. Other public servants are not. This is one of the biggest signals that the US is an oligarchy and not a democracy.